clk: samsung: exynos5420: Define CLK_SECKEY gate clock only or Exynos5420

The bit of GATE_BUS_PERIS1 for CLK_SECKEY is just reserved on
exynos5422/5800, not exynos5420. Define gate clk for exynos5420 to
handle the bit only on exynos5420.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
[m.szyprow: rewrote commit subject]
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Sylwester Nawrocki <snawrocki@kernel.org>