ARM: keystone: defconfig: enable USB support

Enable the USB support (Host mode only) on TI's Keystone platform.
It also enables the support of usb mass storage, FAT and Ext4
filesystems to test rootfs mount over an USB disk.

Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: WingMan Kwok <w-kwok2@ti.com>