Documentation: Add TI TPS6594 PFSM

This patch adds documentation for the TPS6594 PFSM driver.

The PFSM controls the operational states of the TPS6594 PMIC.
Depending on the operational mode, some power domain networks
remain energized while others can be off.

Signed-off-by: Julien Panis <jpanis@baylibre.com>
Message-ID: <20230511095126.105104-6-jpanis@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>