doc: update NFP guide
doc: update NFP guide

New CPP interface changes the way firmware upload is managed by
the PMD. It also supports different firmware file names for
having specific firmware applications per card.

Signed-off-by: Alejandro Lucero <1ffca3a1926aae782ae3c39a5b308878ba1e5683@netronome.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk