mc13xxx: Define maximum SPI clock frequency global to driver
This patch provide setup for SPI clk frequency global to driver.
For MC13783 maximum clock frequency is 20 MHz,
for MC13892 maximum clock frequency is 26 MHz,
so we define 20 MHz as a maximum SPI clk.
Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>