linux/kernel.h: Add PTR_ALIGN_DOWN macro

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.com


Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Rob Herring <robh@kernel.org>