msm: clock-generic: Add get_rate op for ext clocks

The ext clock type may sometimes need to report the
rate of its parent. Add the get_rate op to allow this.

Change-Id: Id22e6a52aa36422fe4583d9c062a962f3399b6c4
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>