arch/arm/cpu/armv7/omap-common/clocks-common.c: Fix build warnings

Fix:
clocks-common.c: In function 'setup_non_essential_dplls':
clocks-common.c:323:6: warning: variable 'sys_clk_khz' set but not used
[-Wunused-but-set-variable]
clocks-common.c: In function 'setup_non_essential_dplls':
clocks-common.c:323:6: warning: variable 'sys_clk_khz' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Tom Rini <trini@ti.com>