fdt_support: fdt_fixup_dr_usb: add support for phy_type fixups

Currently U-Boot can only fixup the usb dr_mode, but some boards (namely
MPC8315E-RDB) can use two PHY types: ULPI (stand-alone OTG port) or UTMI
(connected to the four-ports hub, usb host only).

This patch implements support for passing Dual-Role USB controller's
device tree property phy_type through the usb_phy_type environment
variable.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: Gerald Van Baren <vanbaren@cideas.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>