leds: Add driver for Qualcomm LPG

The Light Pulse Generator (LPG) is a PWM-block found in a wide range of
PMICs from Qualcomm. These PMICs typically comes with 1-8 LPG instances,
with their output being routed to various other components, such as
current sinks or GPIOs.

Each LPG instance can operate on fixed parameters or based on a shared
lookup-table, altering the duty cycle over time. This provides the means
for hardware assisted transitions of LED brightness.

A typical use case for the fixed parameter mode is to drive a PWM
backlight control signal, the driver therefor allows each LPG instance
to be exposed to the kernel either through the LED framework or the PWM
framework.

A typical use case for the LED configuration is to drive RGB LEDs in
smartphones etc, for which the driver supports multiple channels to be
ganged up to a MULTICOLOR LED. In this configuration the pattern
generators will be synchronized, to allow for multi-color patterns.

The idea of modelling this as a LED driver ontop of a PWM driver was
considered, but setting the properties related to patterns does not fit
in the PWM API. Similarly the idea of just duplicating the lower bits in
a PWM and LED driver separately was considered, but this would not allow
the PWM channels and LEDs to be configured on a per-board basis. The
driver implements the more complex LED interface, and provides a PWM
interface on the side of that, in the same driver.

Signed-off-by: Bjorn Andersson <[email protected]>
Tested-by: Douglas Anderson <[email protected]>
Tested-by: Luca Weiss <[email protected]>
Reviewed-by: Marijn Suijten <[email protected]>
Tested-by: Marijn Suijten <[email protected]>
[On the Sony Xperia Nile Discovery, SDM630]
Signed-off-by: Pavel Machek <[email protected]>

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