Add a macro for aligning down a pointer. This is useful to get an
aligned register address when a device allows only word access and
doesn't allow half word or byte access.

Link: https://lore.kernel.org/r/20200722110317.4744-4-kishon@ti.comSigned-off-by: NKishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: NRob Herring <robh@kernel.org>