introduce BIT() definition, used in at91_udc gadget
driver.

Signed-off-by: Heiko Schocher <hs@denx.de>
[remove all other occurrences of BIT(x) definition]
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Acked-by: Stefan Roese <sr@denx.de>
Acked-by: Anatolij Gustschin <agust@denx.de>