ARM: dts: Add missing hwmod related nodes for am33xx

On am33xx we're missing the pmu and emif nodes with their related
"ti,hwmods" properties that the SoC interconnect code needs.

Note that this will only show up as a bug with "doesn't have
mpu register target base" boot errors when the legacy platform
data is removed.

Let's also update the related binding documentation while at it.

Cc: Mark Rutland <mark.rutland@arm.com>
Acked-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>