mfd: rz-mtu3: Replace raw_spin_lock->spin_lock()

As per kernel documentation, use raw_spinlock_t only in real critical core
code, low-level interrupt handling, and places where disabling preemption
or interrupts is required. Here the lock is for concurrent register access
from different drivers, hence spin_lock() is sufficient.

Reported-by: Pavel Machek <pavel@denx.de>
Closes: https://lore.kernel.org/all/ZIL%2FitcJvV5s3Bnf@duo.ucw.cz/


Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Pavel Machek <pavel@denx.de>
Link: https://lore.kernel.org/r/20230815073445.9579-3-biju.das.jz@bp.renesas.com


Signed-off-by: Lee Jones <lee@kernel.org>