Board definition for CH32X033F8P6 TSSOP20

[maxint-rd]

At the current stage it works for me. Have fun!
Info below is also in the readme of the board definition directory.

                  +------v------+
    MISO/A6 D6~ 1-+PA6       PA5+-20  D5 SCK/A5
MOSI/A8/TX4 D7  2-+PA7=PB0   PA4+-19  D4~ CS/A4
     A9/RX4 D8  3-+PB1      PC19+-18  D17 SWCLK
       /RST D9  4-+PB7       PA3+-17  D3~ RX2/A3*
     USBDM D10  5-+PC16=PC11 PA2+-16  D2~ TX2/A2
     USBDP D11  6-+PC17=PC10 PA1+-15  D1~ A1
           GND  7-+VSS       PA0+-14  D0~ A0
     SWDIO D16  8-+PC18      PC3+-13  D15~ A13
           VCC  9-+VDD      PA10+-12  D14/SCL
           D12 10-+PA9      PA11+-11  D13/SDA
                  +-------------+
*A3 and I2C don't work on CH32X033F8P6 0-series (lot number with the penultimate bit 5 being 0).

Tested features

• digitalWrite()/digitalRead() - can use Arduino pin numbers or PAx notation.
• analogWrite() - 12-bit resolution, pins marked with ~ in pinout above.
• analogRead() - very stable 12-bit resolution, but issues with A3/A0/PADC_VREF
• Serial.print() - Tested 115200 bps on PA2/PA3 (as set in variant_CH32X033F8P6.h)
• EEPROM library - may need improvement (first test showed corrupted data written).
• Pin PB7 (Arduino pin 9) can be configured as hardware reset /RST using WCH Link Utility. (See issue Hardware reset pin doesn't work on CH32X033 or CH32X035 #123)

Known issues/limitations

• Pins PA7/PB0, PC16/PC11 and PC17/PC10 cannot be used for output.
• Any signal on A0 seems to show on other ADC pins when disconnected.
• A3, PADC_VREF and I2C don't work on CH32X033F8P6 0-series (lot number with the penultimate bit 5 being 0).