Add new config option and set proper dependencies for ISP.

v2: add missed guards, drop separate Kconfig
Reviewed-by: Pratap Nirujogi <pratap.nirujogi@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Pratap Nirujogi <pratap.nirujogi@amd.com>