Add RZ/G3E xSPI support

Merge series from Biju Das <biju.das.jz@bp.renesas.com>:

The xSPI IP found on RZ/G3E SoC similar to RPC-IF interface, but it
can support writes on memory-mapped area. Even though the registers are
different, the rpcif driver code can be reused for xSPI by adding wrapper
functions.