In ARM, stack decrements before storing values, so decrementing a number
from the stack base is unnecessary. Existing instances of this were
removed in 6b3dc4abd8 ("ARM: Cleanup stack offset cargo cult"), but the
sama5d27-som1 board was added later than that. Fix the cargo cult here
as well and while at it, turn a macro that doesn't need to be one
into a static inline function.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>