rtc: m41t93: migrate to regmap api for register access

rtc: m41t93: migrate to regmap api for register access

Adapt driver to use regmap api with spi bus instead of direct spi
subsystem calls to access device registers. Simplify and standardize the
register interactions using more abstract and bus agnostic regmap api
to reduce code duplication and improve maintainability. Define spi regmap
config suitable for m41t93 spi bus protocol to achieve same transactions on
spi bus.

Tested on TI am62x sk board with m41t93 rtc chip connected over spi0.
Validated set and get time using hwclock tool and verified spi bus
transfers using logic analyzer.

Signed-off-by: Akhilesh Patil <akhilesh@ee.iitb.ac.in>
Link: https://patch.msgid.link/180f9b6c3ee7c490fe3537c2d50a92cec359e4cd.1758379856.git.akhilesh@ee.iitb.ac.in
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>