ARM: at91: dt: enable usb ehci for sam9g45 and sam9x5

make the ECHI depends on ARCH_AT91

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: devicetree-discuss@lists.ozlabs.org