ehci-mx5/6: Make board_ehci_hcd_init() optional

A custom board_ehci_hcd_init() may be unneeded, so add a weak default
implementation doing nothing.

By the way, use simple __weak from linux/compiler.h for
board_ehci_hcd_postinit() instead of weak alias with full attribute.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>