eth fbnic: Add debugfs hooks for firmware mailbox

eth fbnic: Add debugfs hooks for firmware mailbox

This patch adds reporting the Rx and Tx information
interfacing with the firmware.

The result of reading fbnic/fw_mbx is:

 Rx
 Rdy: 1 Head: 11 Tail: 10
 Idx Len  E  Addr        F H   Raw
 ----------------------------------
 00  4096 0 000101fea000 0 1   1000000101fea001
 01  4096 0 000101feb000 0 1   1000000101feb001
 	.
 	.
 	.
 15  4096 0 000101fe9000 0 1   1000000101fe9001

 Tx
 Rdy: 1 Head: 4 Tail: 4
 Idx Len  E  Addr        F H   Raw
 ----------------------------------
 00  0004 1 00010321b000 1 1   000440010321b003
 01  0004 1 00010228d000 1 1   000440010228d003
 	.
 	.
 	.
 15  0004 1 00010321b000 1 1   000440010321b003

Signed-off-by: Mike Marciniszyn (Meta) <mike.marciniszyn@gmail.com>
Link: https://patch.msgid.link/20260127200644.11640-2-mike.marciniszyn@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>