ARM: at91: generic.h: Add include safe guards

ARM: at91: generic.h: Add include safe guards

The generic.h header file doesn't have any safe guards against multiple
inclusion. It only worked so far because all the symbols defined in it were
extern, but this is a rather fragile assumption.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>