serial: samsung: select EXYNOS specific driver data if ARCH_EXYNOS is defined

All EXYNOS4/5 SoCs share a common driver data in the samsung serial
driver. Hence, let the driver data inclusion be based on ARCH_EXYNOS
instead of SOC specific definition.

Signed-off-by: Chander Kashyap <chander.kashyap@linaro.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Reviewed by: Girish K S <ks.giri@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>