ARM: dts: keystone: Add guestos maintenance interrupt

Update the Keystone gic device tree entry to add the maintenance
interrupt information.

Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>