arm64: dts: ti: j721e-main: Add SDHCI nodes

Add nodes for the 3 SDHCI instances present on TI's J721E device.
instance 0 supports HS400 (8 bit bus widht, DDR, 400 MBps)
while instances 1 and 2 support SDR104 (4 bit width, SDR, 100 MBps) as
their highest speed modes. Currently, only High speed (50 MHz clock) has
been enabled.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>