Improve GBA RTC

Implements various improvements to GBA RTC. Primarily, this allows for RTC to actually be reset and set by games, instead of it being forced into matching host time. Time registers are now just incremented as host time increases, like other RTC implementations in mGBA (e.g. MBC3 / HuC3).
"Test mode" is also implemented to the degree it seems to do anything (which just seems to prevent commands from working, except exit test mode and reset commands). It is internally stored in bit 7 of the seconds byte, as documented in spec sheets (although this is impossible to verify it seems, due to get time commands no longer working).
Invalid / write-only commands now return a stream of 1-bits (rather than 0-bits), as on hardware.
The PM flag is now properly implemented (so software can actually use 12 hour mode fully).
Only valid RTC control bits can actually be written now (unused bits are always 0; power-off bit is not settable but it is clearable).
The alarm command is "implemented" here as a stub (the alarm registers are just write only here, so nothing to do without alarms actually being implemented with cartridge IRQ; although I haven't verified yet if cartridge IRQ should actually be triggered or whatever).

Author: CasualPokePlayer

include/mgba/internal/gba/cart/gpio.h

@@ -36,10 +36,22 @@ DECL_BIT(RTCControl, Poweroff, 7);
 
 enum RTCCommand {
 	RTC_RESET = 0,
+	RTC_ALARM = 1,
 	RTC_DATETIME = 2,
-	RTC_FORCE_IRQ = 3,
+	RTC_TEST_START = 3,
 	RTC_CONTROL = 4,
-	RTC_TIME = 6
+	RTC_TIME = 6,
+	RTC_TEST_END = 7
+};
+
+enum RTCDateTime {
+	RTC_YEARS = 0,
+	RTC_MONTHS = 1,
+	RTC_DAYS = 2,
+	RTC_WEEKDAY = 3,
+	RTC_HOURS = 4,
+	RTC_MINUTES = 5,
+	RTC_SECONDS = 6
 };
 
 DECL_BITFIELD(RTCCommandData, uint32_t);
@@ -57,7 +69,6 @@ struct GBARTC {
 	RTCControl control;
 	uint8_t time[7];
 	time_t lastLatch;
-	time_t offset;
 };
 
 DECL_BITFIELD(GPIOPin, uint16_t);
@@ -100,6 +111,8 @@ void GBAHardwareGPIOWrite(struct GBACartridgeHardware* gpio, uint32_t address, u
 void GBAHardwareTiltWrite(struct GBACartridgeHardware* gpio, uint32_t address, uint8_t value);
 uint8_t GBAHardwareTiltRead(struct GBACartridgeHardware* gpio, uint32_t address);
 
+void GBAHardwareRTCSanitize(struct GBACartridgeHardware* gpio);
+
 struct GBASerializedState;
 void GBAHardwareSerialize(const struct GBACartridgeHardware* gpio, struct GBASerializedState* state);
 void GBAHardwareDeserialize(struct GBACartridgeHardware* gpio, const struct GBASerializedState* state);