mmc: sdhci_am654: Add support for clkbuf_sel property

Add support for writing new clkbuf_sel property for the J721e 4 bit IP.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Link: https://lore.kernel.org/r/20200619125801.9530-7-faiz_abbas@ti.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>