spi: bcm-qspi: choose sysclk setting based on requested speed

Check requested speed for a given transfer before setting
27MHz or 108Mhz sysclk on SoCs that support both. This way
for baud rates below 212Khz we can use 27Mhz clock.

Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Link: https://lore.kernel.org/r/20211124193353.32311-2-kdasu.kdev@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>