clk: Export phase functions

The phase setter and getter were not exported until now, which was causing
build breakages when callers were compiled as module. Export these two
functions.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Michael Turquette <mturquette@linaro.org>