Documentation: dt: add the omap hwspinlock bindings document

HwSpinlock IP is present only on OMAP4 and other newer SoCs,
which are all device-tree boot only. This patch adds the
DT bindings information for OMAP hwspinlock module.

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