leds: qcom-lpg: Require pattern to follow documentation

The leds-trigger-pattern documentation describes how the brightness of
the LED should transition linearly from one brightness value to the
next, over the given delta_t.

But the pattern engine in the Qualcomm LPG hardware only supports
holding the brightness for each entry for the period.
This subset of patterns can be represented in the leds-trigger-pattern
by injecting zero-time transitions after each entry in the pattern,
resulting in a pattern that pattern that can be rendered by the LPG.

Rework LPG pattern interface to require these zero-time transitions, to
make it comply with this subset of patterns and reject the patterns it
can't render.

Fixes: 24e2d05d ("leds: Add driver for Qualcomm LPG")
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>