iommu/exynos: Fix trivial typos

Fixed trivial typos and grammar to improve readability.
Changed w/a to workaround.

Signed-off-by: Sachin Kamat <sachin.kamat@samsung.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>