arm64: dts: r8a7795: Add SATA controller node

This adds SATA device node to r8a7795.dtsi.

Signed-off-by: Kouei Abe <kouei.abe.cp@renesas.com>
[uli: adjusted for new MSTP clock scheme]
Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>