mailbox: renesas: Add Renesas MFIS Multifunctional Interface mailbox driver

mailbox: renesas: Add Renesas MFIS Multifunctional Interface mailbox driver

Add support for the Renesas MFIS mailbox, which provides an interface
between the different CPU Cores, such as AP System Core domain and the
Realtime Core domain, SCP Core domain and AP System Core domain or
Realtime Core domain and AP System Core domain or Realtime Core domain.

Signed-off-by: Tuyen Dang <tuyen.dang.xa@renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org> # Update the driver
[Marek: Rename the driver to renesas-mfis, simplify the driver.
        Always use only one TX channel and no RX channel, drop all
	unnecessary code. Perform 1ms delay in send callback which
	is perfectly fine to do in U-Boot which does RX polling]