usb: host: xhci-plat: add support for the R-Car M2-N xHCI controller

This patch adds support for R-Car M2-N (r8a7793) xHCI controller.
This SoC is compatible with R-Car H2 (r8a7790) and R-Car M2-W (r8a7791).

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>