nfp: read ME frequency from vNIC ctrl memory

PCIe island clock frequency is used when converting coalescing
parameters from usecs to NFP timestamps.  Most chips don't run
at 1200MHz, allow FW to provide us with the real frequency.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>