EXYNOS5: FDT: Add compatible strings for Serial

Add required compatible information for s5p serial driver

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Rajeshwari Shinde <rajeshwari.s@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>