mmc: arm_pl180_mmci: Simplify code using mmc_of_parse()

Simplify the code a bit by using the common mmc_of_parse() function
instead of duplicating the device tree parsing code. We can still get
a default value for cfg->f_max by assigning it before calling
mmc_of_parse().

Another advantage of this refactoring is that we parse more properties
now, e.g. "non-removable" can be used to disable CD entirely.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Tested-by: Patrice Chotard <patrice.chotard@foss.st.com> on stm32f769-disco
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>