dt/bindings: mmc: Add bindings for PIC32 SDHCI host controller

Document the devicetree bindings for the SDHCI peripheral found on
Microchip PIC32 class devices.

Signed-off-by: Andrei Pistirica <andrei.pistirica@microchip.com>
Signed-off-by: Joshua Henderson <joshua.henderson@microchip.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>