remoteproc/keystone: Add support for Keystone 66AK2G SOCs

Add support to the keystone remoteproc driver for managing the
DSP present in the Keystone 2 66AK2G SoC. The 66AK2G SoC has
a Power Management Micro Controller (PMMC) that manages the
individual device's power, clock and reset functionalities.

The keystone remoteproc driver already uses standard frameworks
for reset and clock control, so it doesn't require any significant
modifications other than a new compatible suitable for 66AK2G DSP.

The binding document is also updated to reflect the modified
property values used by the 66AK2G DSP node as compared to the
values used by existing Keystone 2 DSPs.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>