dt-bindings: Add the clock id of UART2 and MMC2 for Exynos3250

This patch adds the new clock id for both UART2 and MM2 device
for Exynos3250 SoC.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>