mmc: pic32: Refresh PIC32 MMC driver

mmc: pic32: Refresh PIC32 MMC driver

The existing driver is not compatible with the Driver Model.

This patch makes the necessary changes while also removing obsolescent
calls/properties as follows:

- fdtdec_* calls replaced with dev_read_* equivalents;
- 'clock-freq-min-max' property replaced by querying the frequency of
  the source clock 'base_clk';
- The card detect erratum workaround is applied during probe rather than
  overriding get_cd.

The card detect workaround (Microchip ref. DS80000736E, erratum #15) is
not always needed and can be disabled using a vendor specific DT
property.

Signed-off-by: John Robertson <john.robertson@simiatec.com>