clk: Add CLK_IS_BASIC flag to identify basic clocks

Most platforms end up using a mix of basic clock types and
some which use clk_hw_foo struct for filling in custom platform
information when the clocks don't fit into basic types supported.

In platform code, its useful to know if a clock is using a basic
type or clk_hw_foo, which helps platforms know if they can
safely use to_clk_hw_foo to derive the clk_hw_foo pointer from
clk_hw.

Mark all basic clocks with a CLK_IS_BASIC flag.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>