rtc: rx6110: Constify struct regmap_config

rtc: rx6110: Constify struct regmap_config

'regmap_spi_config' and 'regmap_i2c_config' are not modified in this diver
and are only used as a const struct regmap_config.

Constifying these structures moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig:
Before:
   text	   data	    bss	    dec	    hex	filename
   8896	   1554	     32	  10482	   28f2	drivers/rtc/rtc-rx6110.o

After:
   text	   data	    bss	    dec	    hex	filename
   9536	    914	     32	  10482	   28f2	drivers/rtc/rtc-rx6110.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/833a7f612c0de9dcb1179a0b75b189c237a335ac.1714862560.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>