clk: Remove ifdef for COMMON_CLK in clk-provider.h

This ifdef has been there since the beginning of this file, but it
doesn't really seem to serve any purpose besides obfuscating the struct
definitions and #defines here from compilation units that include it.
Let's always expose these function prototypes and struct definitions so
that code can inspect clk providers without needing to have
CONFIG_COMMON_CLK enabled.

Signed-off-by: Stephen Boyd <sboyd@kernel.org>