clk: renesas: rcar-gen2: Use DEFINE_SPINLOCK() for static spinlock

A static spinlock can be initialized automatically with
DEFINE_SPINLOCK() rather than explicitly calling spin_lock_init().

Suggested-by: Ye Bin <yebin10@huawei.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/8da2c908f00043f05f7e26e3c26400aea0cfe8bc.1716975021.git.geert+renesas@glider.be