net: ftgmac100: Add reset control

Add optional reset control, especially for the Aspeed SOC. For the
hardware without a reset line, the reset assertion/deassertion will be
skipped.

Signed-off-by: Dylan Hung <dylan_hung@aspeedtech.com>
Reviewed-by: Ryan Chen <ryan_chen@aspeedtech.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>