dt-bindings: mfd: Add NXP BBNSM

Add binding for NXP BBNSM(Battery-Backed Non-Secure Module).

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230129070823.1945489-2-ping.bai@nxp.com