cxgb4: Add functions to read memory via PCIE memory window

This patch implements two new functions t4_mem_win_read and t4_memory_read.
These new functions can be used to read memory via the PCIE memory window.
Please note, for proper execution of these functions PCIE_MEM_ACCESS_BASE_WIN
registers must be setup correctly like how setup_memwin in the cxgb4 driver
does it.

Signed-off-by: Jay Hernandez <jay@chelsio.com>
Signed-off-by: Vipul Pandya <vipul@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>