ARM: EXYNOS: Select SPARSE_IRQ for Exynos

This patch adds selection of CONFIG_SPARSE_IRQ for ARCH_EXYNOS, since it
is required by multiplatform and allows to remove the legacy mach/irqs.h
header.

To make this possible, a dummy IRQ_EINT_BIT macro is added to pm-core.h
header to allow plat-samsung/pm.c compile. This macro is irrelevant for
Exynos and will be removed after reworking Samsung pm code for
multiplatform compatibility.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>