net: lan966x: Add PGID_GP_START and PGID_GP_END

The first entries in the PGID table are used by the front ports while
the last entries are used for different purposes like flooding mask,
copy to CPU, etc. So add these macros to define which entries can be
used for general purpose.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>