rtc: add dm_rtc_write() helper

rtc: add dm_rtc_write() helper

Similar to dm_rtc_read(), introduce a helper that allows the caller to
write multiple consecutive 8-bit registers with one call. If the
driver provides the ->write method, use that, otherwise loop using
->write8.

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