clk: samsung: exynos4: add missing smmu_g2d clock and update comments

This patch adds missing smmu_g2d clock implementation and updates
comment about Exynos4 clocks from 278-282 range. Those clocks are
available on all Exynos4 SoC series, so the misleading comment has been
removed.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>