ehci-exynos: Allow to specify clken_polarity from env

ODROID-X2 boards expect active high to enable 24Mhz clock
This patch introduces 'usb_invert_clken' environment variable
that does the trick.
For X2 boards - set to 1, for U/U2 - set to 0 (default)

Signed-off-by: Andrew Andrianov <andrew@ncrmnt.org>