clk/samsung: exynos542x: mark some clocks as critical

Some parent clocks of the Exynos542x clock blocks, which have separate
power domains (like DISP, MFC, MSC, GSC, FSYS and G2D) must be always
enabled to access any register related to power management unit or devices
connected to it. For the time being, until a proper solution based on
runtime PM is applied, mark those clocks as critical (instead of ignore
unused or even no flags) to prevent disabling them.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Tested-by: Javier Martinez Canillas <javier@osg.samsung.com> [Exynos5800 Peach Pi Chromebook]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>