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

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>