s5j/serial: add clk gating condition
s5j/serial: add clk gating condition

Clock gating should only work when PM is supported.

Change-Id: I7de7251241e7f2ec3189f65f61db8057eca2fd3e
Signed-off-by: Junhwan Park <7fee488b6f78fc50b291ffc7d10428788977e877@samsung.com>

JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT