octeontx2-af: Cleanup CGX config permission checks

Most of the CGX register config is restricted to mapped RVU PFs,
this patch cleans up these permission checks spread across
the rvu_cgx.c file by moving the checks to a common fn().

Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>