nfp: bpf: read from packet data cache for PTR_TO_PACKET

This patch assumes there is a packet data cache, and would try to read
packet data from the cache instead of from memory.

This patch only implements the optimisation "backend", it doesn't build
the packet data cache, so this optimisation is not enabled.

This patch has only enabled aligned packet data read, i.e. when the read
offset to the start of cache is REG_WIDTH aligned.

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