dt-bindings: mfd: Add TI TPS6594 PMIC

TPS6594 is a Power Management IC which provides regulators and others
features like GPIOs, RTC, watchdog, ESMs (Error Signal Monitor), and
PFSM (Pre-configurable Finite State Machine) managing the state of the
device.
TPS6594 is the super-set device while TPS6593 and LP8764 are derivatives.

Signed-off-by: Julien Panis <jpanis@baylibre.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230511095126.105104-2-jpanis@baylibre.com