remoteproc/keystone: drop ti,syscon-psc fallback support

The Keystone remoteproc driver has already been enhanced to
support using the reset framework API for managing the DSP
device resets. All the existing DSP DT nodes have also been
switched to use the 'resets' property instead of the deprecated
'ti,syscon-psc' property.

All the Keystone 2 family of SoCs are expected to be using reset
drivers going forward, so drop the support for the direct syscon
PSC based reset handling from the driver completely. The DT
binding has also been updated accordingly.

While at this, also add a build dependency for the driver
against the RESET_CONTROLLER Kconfig, and switch to using the
devm_reset_control_get() API instead of the optional variant.

Signed-off-by: Andrew F. Davis <afd@ti.com>
[s-anna@ti.com: mandatory reset dependencies, binding updates]
Signed-off-by: Suman Anna <s-anna@ti.com>