pinctrl: dt-bindings: Add dt-bindings for clk-names and external clk

Add lpi clock-names and clocks properties to dt-bindings for pinctrl.

Change-Id: I81b4891e85749111ce3e013aa7f83fba3a6c2157
Signed-off-by: Vatsal Bucha <vbucha@codeaurora.org>