ARM: dts: use macro in gpio keys for exynos3250 boards

This patch replaces number by macro in gpio keys for exynos3250 boards.

Cc: Youngjun Cho <yj44.cho@samsung.com>
Cc: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Beomho Seo <beomho.seo@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>