MIPS: ralink: mt7620: Add spi clock definition

Register a clock device for the SPI block of the
MT7620 SoC. The clock device will be used by the
SPI host controller driver to determine the base
clock of the controller.

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5754/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>