net: mdio: add reset control for Aspeed MDIO

Add reset assertion/deassertion for Aspeed MDIO.  There are 4 MDIO
controllers embedded in Aspeed AST2600 SOC and share one reset control
register SCU50[3].  To work with old DT blobs which don't have the reset
property, devm_reset_control_get_optional_shared is used in this change.

Signed-off-by: Dylan Hung <dylan_hung@aspeedtech.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>