mmc: sdhci-xenon: use clk only with DT

As a preparation for supporting ACPI, modify the driver
to use the clk framework only when booting with DT -
otherwise rely on the configuration done by firmware.
For that purpose introduce also a custom SDHCI get_max_clock
callback.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Link: https://lore.kernel.org/r/20201204171626.10935-4-mw@semihalf.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>