mlxsw: Add 'ipv4_5' flex key

Currently virtual router ID element is broken to two sub-elements -
'VIRT_ROUTER_LSB' and 'VIRT_ROUTER_MSB'. It was broken as this field is
broken in 'ipv4_4' flex key which is used for IPv4 in Spectrum < 4.
For Spectrum-4, we use 'ipv4_4b' flex key which contains one field for
virtual router, this key is not supported in older ASICs.

Add 'ipv4_5' flex key which is supported in all ASICs and contains one
field for virtual router. Then there is no reason to use 'VIRT_ROUTER_LSB'
and 'VIRT_ROUTER_MSB', remove them and add one element 'VIRT_ROUTER' for
this field.

The motivation is to get rid of 'ipv4_4' flex key, as it might be chosen
for IPv6 multicast forwarding region. This will not allow the improvement
in a following patch. See more details in the cover letter and in a
following patch.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>