ARM: davinci: da8xx: Add USB PHY platform device

There is now a proper phy driver for the DA8xx SoC USB PHY. This adds the
platform device declarations needed to use it.

Signed-off-by: David Lechner <david@lechnology.com>
[nsekhar@ti.com: keep usb-davinci.h included in board-da830-evm.c
		 minor subject line adjustment]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>