tests: drivers: reset: mmio: keep the fake registers out of kernel RAM

The qemu_cortex_m3 overlay placed the fake reset-mmio registers at
0x20000004/0x20000008, inside the SRAM the kernel links into. The
words they occupy are owned by whatever .data the linker places at
the start of RAM, so every write through the reset driver corrupts
live kernel state, and the test only ever passed by layout luck.

A recent timer-core change added an 8-byte .data variable that landed
at 0x20000000, which pushed libc's _char_out console output function
pointer to exactly 0x20000008: the active-low test case then wrote
its register patterns over the pointer, console output jumped through
the corrupted value into a HardFault, the fault handler's own print
re-took the fault, and QEMU stopped with "Lockup: can't escalate 3 to
HardFault". Any future .data layout change could break this test the
same way.

Shrink the kernel's view of the 64 KiB SRAM by one KiB in the overlay
and move the fake registers into the carved-out tail, so no linker
layout can ever place kernel data under them.

Assisted by Claude (claude-opus-4-8).
Signed-off-by: Anas Nashif <anas.nashif@intel.com>

Author: nashif

tests/drivers/reset/mmio/boards/qemu_cortex_m3.overlay

@@ -1,13 +1,31 @@
+/*
+ * Copyright (c) 2025 Google LLC.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/*
+ * The fake reset registers below are backed by plain SRAM, so the words
+ * they occupy must not be part of the RAM the kernel links into: a write
+ * through the reset driver would clobber whatever data or function
+ * pointer the linker happened to place there (moving with every layout
+ * change). Shrink the kernel's view of the 64 KiB SRAM by one KiB and
+ * place the registers in the carved-out tail.
+ */
+&sram0 {
+	reg = <0x20000000 0xfc00>;
+};
+
 / {
-	reset0: reset@20000004 {
+	reset0: reset@2000fc00 {
 		compatible = "reset-mmio";
-		reg = <0x20000004 0x4>;
+		reg = <0x2000fc00 0x4>;
 		num-resets = <16>;
 	};
 
-	reset1: reset@20000008 {
+	reset1: reset@2000fc04 {
 		compatible = "reset-mmio";
-		reg = <0x20000008 0x4>;
+		reg = <0x2000fc04 0x4>;
 		num-resets = <16>;
 		active-low;
 	};