ARM: dts: keystone: specify usb mode explicitly

The USB OTG mode is not supported by Kestone 2 devices, as result, the
USB devices enumeration and detection will not work properly when
kernel is built with CONFIG_USB_DWC3_DUAL_ROLE=y (default for multi
platform build):
 - it's required to load gadget drivers manually to make host mode
work and this confuses current Keystone 2 users
 - device mode is not working, because port can't detect and switch to
peripheral/host mode dynamically.

Hence, specify usb mode explicitly in DT: usb0 = "host" for all KS2
devices and usb1 = "peripheral" for K2E.

Cc: Sekhar Nori <nsekhar@ti.com>
Cc: Roger Quadros <rogerq@ti.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>