mmc: am654_sdhci: Add support for writing to clkbuf_sel

Add support for writing new clock buffer select property for both
the am654x and j721e 4 bit IPs

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>