clk: samsung: exynos4: remove duplicate div_core2 divider clock instantiation

The 'div_core2' clock and the 'arm_clk' divider clocks are instances of
the same divider clock. So remove the 'arm_clk' clock instance.

Signed-off-by: Thomas Abraham <thomas.ab@samsung.com>
[tomasz.figa@gmail.com: Fixed remaining occurences of 'arm_clk'.]
Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>