clk: Allow phase adjustment from debugfs

For testing it may be useful to manually adjust a clock's phase.  Add
support for writing to the existing clk_phase debugfs file, with the
written value clamped to [0, 360) to match the behaviour of the
clk_set_phase() function.

This is a dangerous feature, so use the existing define
CLOCK_ALLOW_WRITE_DEBUGFS to allow it only if the source is modified.

Signed-off-by: John Keeping <john@metanate.com>
Link: https://lore.kernel.org/r/20230420103805.125246-1-john@metanate.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>