board: rzg2l: Support reset via Renesas RAA215300 PMIC

With the SYSRESET subsystem enabled we do not need to implement
reset_cpu() in the board directory.

Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>