dt-bindings: arm-smmu: add compatible string for Marvell Armada-AP806 SMMU-500

Add specific compatible string for Marvell usage due to errata of
accessing 64bits registers of ARM SMMU, in AP806.

AP806 SoC uses the generic ARM-MMU500, and there's no specific
implementation of Marvell, this compatible is used for errata only.

Signed-off-by: Hanna Hawa <hannah@marvell.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Tomasz Nowicki <tn@semihalf.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/20200715070649.18733-4-tn@semihalf.com
Signed-off-by: Will Deacon <will@kernel.org>