ARM: am3517-evm: Disable CONFIG_USB_EHCI_OMAP in SPL

Found accidentally in omap3_logic, CONFIG_USB_EHCI_OMAP adds some
code size to SPL, so this patch disables it on the am3517-evm to
reduce the code a bit since it's tight for space.

Signed-off-by: Adam Ford <aford173@gmail.com>