net: thunderx: Enable mailbox interrupts on 81xx/83xx

88xx has 128 VFs, 81xx has 8 VFs and 83xx will have 32VFs.
Made changes to PF driver such that mailbox interrupt enable
registers are configuired based on number of VFs HW supports.
Also cleanedup mailbox irq handler registration code.

Signed-off-by: Sunil Goutham <sgoutham@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>