exynos: fix header inclusion guard

It seems like the header inclusion guard for the Exynos pinctrl header
was misspelled.

Make the preprocessor symbol for the #ifndef and #define lines the
same, so that the double inclusion protection works as expected.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Tom Rini <trini@konsulko.com>