pmic: Add Renesas RAA215300 PMIC driver

The RZ/G2L SMARC module is powered via a Renesas RAA215300 PMIC which
provides several voltage converters, a real time clock (RTC) and reset
control. A basic driver is implemented for this device so that we can
read, write and dump the PMIC registers.

The raa215300_bind() function is added as a stub, binding of the
sysreset driver will be added in a later patch.

Additional features of this PMIC (such as reset control) may be
supported by future patches.

Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>