serial: samsung: Fix serial config dependencies for exynos7

Exynos7 has a similar serial controller to that present in older Samsung
SoCs. To re-use the existing serial driver on Exynos7 we need to have
SERIAL_SAMSUNG_UARTS_4 and SERIAL_SAMSUNG_UARTS selected. This is not
possible because these symbols are dependent on PLAT_SAMSUNG which is
not present for the ARMv8 based exynos7.

Change the dependency of these symbols from PLAT_SAMSUNG to the serial
driver thus making it available on exynos7. As the existing platform
specific code making use of these symbols is related to uart driver this
change in dependency should not cause any issues.

Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Signed-off-by: Naveen Krishna Chatradhi <ch.naveen@samsung.com>
Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>