iommu/exynos: Use the devm_clk_get_optional() helper

Use devm_clk_get_optional() instead of hand writing it.
This saves some loC and improves the semantic.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/99c0d5ce643737ee0952df41fd60433a0bbeb447.1679834256.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Joerg Roedel <jroedel@suse.de>