nfp: bpf: add support for reading map memory

Map memory needs to use 40 bit addressing.  Add handling of such
accesses.  Since 40 bit addresses are formed by using both 32 bit
operands we need to pre-calculate the actual address instead of
adding in the offset inside the instruction, like we did in 32 bit
mode.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>