Create a core function for set_phase, as it is done for set_rate and
set_parent.

This rework is done to ease the integration of "protected" clock
functionality.
Acked-by: NLinus Walleij <linus.walleij@linaro.org>
Tested-by: NQuentin Schulz <quentin.schulz@free-electrons.com>
Tested-by: NMaxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: NMichael Turquette <mturquette@baylibre.com>
Signed-off-by: NJerome Brunet <jbrunet@baylibre.com>
Signed-off-by: NMichael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20171201215200.23523-4-jbrunet@baylibre.com