clk: ti: export the omap2_clk_is_hw_omap call

There is one instance outside the TI clock driver that needs the info
whether a clock is an OMAP HW clock or not. Thus, move the function
declaration into the public header.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>