mfd: Add Renesas RZ/G2L MTU3a core driver

The RZ/G2L multi-function timer pulse unit 3 (MTU3a) is embedded in
the Renesas RZ/G2L family SoCs. It consists of eight 16-bit timer
channels and one 32-bit timer channel. It supports the following
functions
- Counter
- Timer
- PWM

The 8/16/32 bit registers are mixed in each channel.

Add MTU3a core driver for RZ/G2L SoC. The core driver shares the
clk and channel register access for the other child devices like
Counter, PWM and Clock event.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230330111632.169434-3-biju.das.jz@bp.renesas.com