ARM: dts: mdm9640: Enable RPM notification

Re-enable RPM notification bit support.
MDM9640 has SLP_CMD_MODE that could be set for rpm notification.

This was reverted beacuse of system hang in power collapse
which is resolved with toggling of hardware axi clock gating
over power collapse and with updated spm sequence.

Also, update spm command accordingly.

Change-Id: I2aff4d65356ed4b4a4cff44ab4961110a9660101
Signed-off-by: Girish S Ghongdemath <girishsg@codeaurora.org>