dt-bindings: pinctrl: spacemit: add syscon property

dt-bindings: pinctrl: spacemit: add syscon property

In order to access the protected IO power domain registers, a valid
unlock sequence must be performed by writing the required keys to the
AIB Secure Access Register (ASAR).

The ASAR register resides within the APBC register address space.
A corresponding syscon property is added to allow the pinctrl driver
to access this register.

Signed-off-by: Troy Mitchell <troy.mitchell@linux.spacemit.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Linus Walleij <linusw@kernel.org>