ARM: EXYNOS: use static in suspend.c

The 'pm_data', 'exynos_release_ret_regs', 'exynos3250_release_ret_regs'
and 'exynos5420_release_ret_regs' are not exported nor used outside of
suspend.c file. Make them static.

This fixes following sparse warnings:
arch/arm/mach-exynos/suspend.c:83:23: warning: symbol 'pm_data' was not declared. Should it be static?
arch/arm/mach-exynos/suspend.c:106:14: warning: symbol 'exynos_release_ret_regs' was not declared. Should it be static?
arch/arm/mach-exynos/suspend.c:117:14: warning: symbol 'exynos5420_release_ret_regs' was not declared. Should it be static?

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>