ARM: dts: AM33XX: Add SHAM data and documentation

Add the generic AM33XX SHAM module's device tree data and
enable it for the am335x-evm, am335x-evmsk, and am335x-bone
platforms.  Also add Documentation file describing the data
for the SHAM module.

Cc: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Mark A. Greer <mgreer@animalcreek.com>
Signed-off-by: Joel Fernandes <joelf@ti.com>
[joelf@ti.com: Dropped interrupt-parent property, documentation fixups]
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Benoit Cousson <bcousson@baylibre.com>