iommu/exynos: Make driver independent of the system page size

PAGE_{SIZE,SHIFT} macros depend on the configured kernel's page size, but
the driver expects values calculated as for 4KB pages. Fix this.

Reported-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220623093629.32178-1-m.szyprowski@samsung.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>