mmc: zynq: parse dt when probing

Currently, the entry "bus-width = <8>" in the ZynqMP's sdhci nodes
is not evaluated. This results in the bus width staying at its default
value (4 bit in HS200 mode).
Fix this by calling mmc_of_parse. This function also checks for the
"no-1-8-v" and "max-frequency" entries. Remove the handling of those
nodes from this driver.

Signed-off-by: Benedikt Grassl <Benedikt.Grassl@rohde-schwarz.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>