remoteproc: k3-r5: Implement is_running operation

remoteproc: k3-r5: Implement is_running operation

Add is_running callback to query the R5F core halt status via the
TI-SCI processor control API. This allows the remoteproc framework
to determine whether the R5F core is currently runnin.

The core is considered running when the PROC_BOOT_CTRL_FLAG_R5_CORE_HALT
bit is not set in the control flags.

Signed-off-by: Philippe Schenker <philippe.schenker@impulsing.ch>
Reviewed-by: Andrew Davis <afd@ti.com>