soc: mediatek: PMIC wrap: Fix clock rate handling

replace chipselect extension values based on SPI clock with hardcoded SoC
specific values.

The PMIC wrapper has the ability of extending the chipselects by configurable
amounts of time. We configured the values based on the rate of SPI clock, but
this is wrong. The delays should be configured based on the internal PMIC clock
that latches the values from the SPI bus to the internal PMIC registers. By
default this clock is 24MHz. Other clock frequencies are for debugging only
and can be removed from the driver.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>