Documentation: dt: add common bindings for hwspinlock

This patch adds the generic common bindings used to represent
a hwlock device and use/request locks in a device-tree build.

Each hwspinlock provider should have the '#hwlock-cells' property,
which represents the number of cells to be used for representing
a specific hwspinlock. Client users shall use the property
'hwlocks' for requesting specific lock(s).

Note that the document is named hwlock.txt deliberately to keep
it a bit more generic.

Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Suman Anna <s-anna@ti.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>