misc: tps6594-pfsm: Add driver for TI TPS6594 PFSM

This PFSM controls the operational modes of the PMIC:
- STANDBY and LP_STANDBY,
- ACTIVE state,
- MCU_ONLY state,
- RETENTION state, with or without DDR and/or GPIO retention.
Depending on the current operational mode, some voltage domains
remain energized while others can be off.

This PFSM is also used to trigger a firmware update, and provides
R/W access to device registers.

See Documentation/misc-devices/tps6594-pfsm.rst for more
information.

Signed-off-by: Julien Panis <jpanis@baylibre.com>
Message-ID: <20230511095126.105104-5-jpanis@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>