pwm: samsung: Allow Samsung PWM driver to be enabled on Exynos7

To re-use the existing PWM driver for 64-bit ARM based Exynos7 SoC, make
the driver depend on ARCH_EXYNOS along with PLAT_SAMSUNG.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>