mtd: rpc: Add Renesas RPC Hyperflash driver

Add driver for the RPC block in Hyperflash mode. This driver allows
access to a CFI Hyperflash attached to the RPC block and does not
support RPC in SPI mode.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>