mmc: stm32_sdmmc2: Use mmc_of_parse() to read host capabilities

mmc_of_parse() can populate the 'f_max' and 'host_caps' fields of
struct mmc_config from devicetree.
The same logic is duplicated in stm32_sdmmc2_probe(). Use
mmc_of_parse(), which is more generic.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>