rtc: rzn1: reduce register access

This RTC has special 32bit registers which return multiple of the same
8bit registers at once. Use these to minimize register access. Also, do
the to/from BCD conversions right away, so 'tm' always contains values
as described in time.h.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20241122101448.4374-3-wsa+renesas@sang-engineering.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>