boards: arm64: Add networking support to qemu_cortex_a53

Enable qemu_cortex_a53 networking using the Intel e1000 Ethernet driver.
This board only supports a single UART, so subsystems like logging and
shell would need to be disabled to use SLIP, therefore QEMU Ethernet is
used as the default instead.

Signed-off-by: Grant Ramsay <gramsay@enphaseenergy.com>