rtc: rx6110: declare missing of table

Add missing <of_device_id> table for SPI driver relying on SPI
device match since compatible is in a DT binding or in a DTS.

Before this patch:
modinfo drivers/rtc/rtc-rx6110.ko | grep alias
alias:          spi:rx6110

After this patch:
modinfo drivers/rtc/rtc-rx6110.ko | grep alias
alias:          spi:rx6110
alias:          of:N*T*Cepson,rx6110C*
alias:          of:N*T*Cepson,rx6110

Reported-by: Javier Martinez Canillas <javier@dowhile0.org>
Signed-off-by: Daniel Gomez <dagmcr@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>