mfd: tps65912-spi: Add missing of table registration

MODULE_DEVICE_TABLE(of, <of_match_table> should be called to complete DT
OF mathing mechanism and register it.

Before this patch:
modinfo drivers/mfd/tps65912-spi.ko | grep alias
alias:          spi:tps65912

After this patch:
modinfo drivers/mfd/tps65912-spi.ko | grep alias
alias:          of:N*T*Cti,tps65912C*
alias:          of:N*T*Cti,tps65912
alias:          spi:tps65912

Reported-by: Javier Martinez Canillas <javier@dowhile0.org>
Signed-off-by: Daniel Gomez <dagmcr@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>