nfp: bpf: add support for bpf_get_prandom_u32()

NFP has a prng register, which we can read to obtain a u32 worth
of pseudo random data.  Generate code for it.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>