test: dm: rtc: add test of dm_rtc_read, dm_rtc_write

Define a few aux registers and check that they can be read/written
individually. Also check that one can access the time-keeping
registers directly and get the expected results.

Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>

arch/sandbox/include/asm/rtc.h

@@ -21,6 +21,11 @@ enum {
 
 	REG_RESET	= 0x20,
 
+	REG_AUX0	= 0x30,
+	REG_AUX1,
+	REG_AUX2,
+	REG_AUX3,
+
 	REG_COUNT	= 0x80,
 };