Introduce new driver for NXP Ethernet controller

ENETC it a gigabit Ethernet controller found on the LS1028A board.
It supports basic VLAN offloads - tag extraction, injection and hardware
filtering. Inband MDIO connectivity is used for link status
monitoring through the miibus interface. Fixed-link mode is also
supported, which allows for operation of internal cpu to switch port.
Since no admin interrupts are present in hardware, link status polling
has to be used.
Due to a hardware bug software reset of the NIC results in a external
abort. Because of that most of the hardware initialization is done
during attach. This also means that in the case of an fatal error full
board reset is required.
The enetc_hw.h header was imporoted from Linux. It is dual licensed.

Submitted by: Kornel Duleba <mindal@semihalf.com>
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D30729