pinctrl: renesas: rzv2m: Fix NULL pointer dereference in rzv2m_dt_subnode_to_map()

Fix the below random NULL pointer crash during boot by serializing
pinctrl group and function creation/remove calls in
rzv2m_dt_subnode_to_map() with mutex lock.

Crash logs:
    pc : __pi_strcmp+0x20/0x140
    lr : pinmux_func_name_to_selector+0x68/0xa4
    Call trace:
    __pi_strcmp+0x20/0x140
    pinmux_generic_add_function+0x34/0xcc
    rzv2m_dt_subnode_to_map+0x2e4/0x418
    rzv2m_dt_node_to_map+0x15c/0x18c
    pinctrl_dt_to_map+0x218/0x37c
    create_pinctrl+0x70/0x3d8

While at it, add a comment for lock.

Fixes: 92a9b825

 ("pinctrl: renesas: Add RZ/V2M pin and gpio controller driver")
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20230815131558.33787-3-biju.das.jz@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>