ARM: dts: Aspeed: AST2600: Add XDMA PCI-E root control reset

The AST2600 XDMA engine requires the PCI-E root control reset be cleared
as well, so add a phandle to that syscon reset.

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