rtc: rk808: reduce 'struct rk808' usage

rtc: rk808: reduce 'struct rk808' usage

Reduce usage of 'struct rk808' (driver data of the parent MFD), so
that only the chip variant field is still being accessed directly.
This allows restructuring the MFD driver to support SPI based
PMICs.

Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20221020204251.108565-4-sebastian.reichel@collabora.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>