Documentation: dt-bindings: update xhci-platform DT binding for R-Car H2 and M2

This commit extends the compatible string list of the xhci-platform
binding with the new "renesas,xhci-r8a7790" and "renesas,xhci-r8a7791"
compatible strings. It is used to describe the xHCI controller which
is available in the R-Car H2 and M2 SoCs.

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>