rtc: davinci: replace 32bit access with 8bit access

Use 32-bit access only where it is needed. Most of the RTC registers
contain useful information in the 8 least significant bits, the others
are reserved.

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210602203805.11494-4-dariobin@libero.it