Qualcomm SoCs based on the RPM architecture have a MSM Power Manager (MPM)
in always-on domain. In addition to managing resources during sleep, the
hardware also has an interrupt controller that monitors the interrupts
when the system is asleep, wakes up the APSS when one of these interrupts
occur and replays it to GIC after it becomes operational.

It adds an irqchip driver for this interrupt controller, and here are
some notes about it.

- For given SoC, a fixed number of MPM pins are supported, e.g. 96 pins
  on QCM2290.  Each of these MPM pins can be either a MPM_GIC pin or
  a MPM_GPIO pin. The mapping between MPM_GIC pin and GIC interrupt
  is defined by SoC, as well as the mapping between MPM_GPIO pin and
  GPIO number.  The former mapping is retrieved from device tree, while
  the latter is defined in TLMM pinctrl driver.

- The power domain (PD) .power_off hook is used to notify RPM that APSS
  is about to power collapse.  This requires MPM PD be the parent PD of
  CPU cluster.

- When SoC gets awake from sleep mode, the driver will receive an
  interrupt from RPM, so that it can replay interrupt for particular
  polarity.
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220308080534.3384532-3-shawn.guo@linaro.org