mmc: sdhci_am654: Read ti, strobe-sel property from device tree

Read the strobe select value from the device tree property ti,strobe-sel,
required for HS400 speed mode

Fixes: a20008ea ("mmc: am654_sdhci: Add Support for configuring PHY in J721e")
Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Link: https://lore.kernel.org/r/20210525093826.10390-2-a-govindraju@ti.com