clk: renesas: cpg-lib: Use DEFINE_SPINLOCK() for global spinlock

A global 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/9073a6bfb7791e492156331fa8a0ea87a7c7cef6.1716975021.git.geert+renesas@glider.be