msm: clock-generic: Introduce an ext_clk clock type

Some clock controllers have input clock signals from
other clock controllers or hardware blocks. To model
these exterior signals, introduce an ext_clk type that
just passes requests to its parent clock. The clock
controller using the ext_clk type will have to do a
clk_get to get a handle to the external clock, and set
that as the parent on the ext_clk clock.

Change-Id: I762f7b4663d5a55d95e5d3b6808004b77b58e2da
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>