clk: renesas: cpg-mssr: Rename cpg_mssr_priv.mstp_lock

The spinlock is used to protect Read-Modify-Write register accesses,
which won't be limited to SMSTPCR register accesses.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>