arm64: dts: r8a7795: salvator-x: enable SDHI0 & 3

Add the exposed SD card slots. The on-board eMMC needs to wait until we
fixed the 8bit support.

Signed-off-by: Ai Kyuse <ai.kyuse.uw@renesas.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>