doc: update thunderx driver guide
doc: update thunderx driver guide

Updating the thunderx with loopback port support,
debugging options, new features added and correcting
some formatting issues.

Signed-off-by: Harman Kalra <568a3484dfecf16d6b30cd214c570848bb408570@marvell.com>
Acked-by: Jerin Jacob <352c4d4f9291b869992ddb67daa45ddc149fed30@marvell.com>

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