83xx: mpc8315erdb: add support for switching between ULPI/UTMI USB PHYs

Freescale ships MPC8315E-RDB boards either with TSEC1 and USB UTMI
support, or without TSEC1 but with USB ULPI PHY support in addition.
With this patch user can specify desired USB PHY.

Also, it seems that we can't distinguish the two boards in software, so
user have to set `mpc8315erdb' environment variable to either 'tsec1'
(TSEC1 enabled) or `ulpi' (board with ULPI PHY, TSEC1 disabled), so that
Linux will not probe for TSEC1.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>