msm: clock-local: Make .is_local assignments optional

Rather than forcing the assignment of the is_local clock op for all clock
types, consider clocks with a NULL is_local function pointer as local
by default. This allows for the removal of local_clk_is_local() and
dummy_clk_is_local(), which always returned true.

Change-Id: Ib5f275acf3dec5c2a0e57243d32dd357b936d26b
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>