bug.h: introduce WARN_ONCE

Add WARN_ONCE definition to allow single time notification
of warnings to the user.
Taken from Linux kernel (4.17) with slight changes
(Removed __section(.data.once))

Signed-off-by: Ramon Fried <ramon.fried@gmail.com>
[trini: Drop the musb and dwc3 compat versions]
Signed-off-by: Tom Rini <trini@konsulko.com>