USB: Check for ARCH_EXYNOS separately

ARCH_EXYNOS is going to be excluded from PLAT_S5P, so it must be checked
separately in Exynos-related Kconfig entries.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>