net: thunderx: Introduce BGX_ID_MASK macro to extract bgx_id

This patch fixes the 'bgx_id' determination on 83xx where there are
4 BGX blocks instead of 2 on other platforms.

Signed-off-by: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>
Signed-off-by: Sunil Goutham <sgoutham@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>