ARM: dts: keystone-k2e: Move keystone_dsp_gpio to under device-state-control

The keystone_irq node describes a device that is a member of the device
state control module address space. As such, it should not be a member
of soc0 bus but instead a sub-node of device-state-control.

This move also fixes a warning about not having a reg property. Now
that this is a sub-node of device-state-control, a syscon type node,
we add this reg property but relative to the syscon base, this way
when the dt-binding/driver are updated we can drop the non-standard
gpio,syscon-dev property completely and simply use get_resource() in
the driver.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>