mfd: intel-m10-bmc: Expose MAC address and count

Create two sysfs entries for exposing the MAC address and count
from the MAX10 BMC register space. The MAC address is the first
in a sequential block of MAC addresses reserved for the FPGA card.
The MAC count is the number of MAC addresses in the reserved block.

Signed-off-by: Russ Weight <russell.h.weight@intel.com>
Signed-off-by: Xu Yilun <yilun.xu@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>