reset: Add of_reset_control_get_optional_exclusive()

Add optional variant of of_reset_control_get_exclusive(). If the
requested reset is not specified in the device tree, this function
returns NULL instead of an error.

Suggested-by: Philipp Zabel <p.zabel@pengutronix.de>
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/20211112184413.4391-2-biju.das.jz@bp.renesas.com


Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>