ARM: dts: Aspeed: tacoma: Enable EHCI controller

Enable the second EHCI controller on the AST2600. Also add a line-name
for the GPIO that controls power to the USB port.

The power control is in place to allow the port to be disabled, for
those that are worried about rogue USB sticks.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>