iommu/sun50i: Use __GFP_ZERO instead of memset()

Allocate zeroed memory so there is no need to memset it to 0 in the
driver.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Cc: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20200514124621.25999-2-joro@8bytes.org