powerpc/83xx: Do not configure or probe disabled FSL DR USB controllers

On MPC837X CPUs Dual-Role USB isn't always available (for example DR
USB pins can be muxed away to eSDHC).

U-Boot adds status = "disabled" property into the DR USB nodes to
indicate that we must not try to configure or probe Dual-Role USB,
otherwise we'll break eSDHC support on targets with MPC837X CPUs.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>