USB: MX5: add generic USB EHCI support for mx51 and mx53

It's derived from ehci-mxc and uses the header files of the
ehci-fsl interface. The callback board_ehci_hcd_init() has
been introduced to allow for board-specific setup when USB
is started.

Signed-off-by: Wolfgang Grandegger <wg@denx.de>
CC: Stefano Babic <sbabic@denx.de>
CC: Remy Bohmer <linux@bohmer.net>