USB: EHCI: Allow EHCI post-powerup configuration in board files

This patch allows USB to work on some hosts, which need additional frobing after
the host was powered up via regular USB powerup sequence.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Remy Bohmer <linux@bohmer.net>
Cc: Wolfgang Grandegger <wg@denx.de>
Cc: Jason Liu <r64343@freescale.com>