nfp: bpf: use the power of sparse to check we encode registers right

Define a new __bitwise type for software representation of registers.
This will allow us to catch incorrect parameter types using sparse.

Accessors we define also allow us to return correct enum type and
therefore ensure all switches handle all register types.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>