dt-bindings: reset: rtd1295: Add SB2 reset

Add a constant for reset3 SB2, based on downstream crt_sys_reg.h.

Acked-by: Rob Herring <robh@kernel.org>
Acked-by: James Tai <james.tai@realtek.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>