dt-bindings: power: supply: sc27xx-fg: add low voltage alarm IRQ

The SC27XX fuel gauge supports a low voltage alarm IRQ, which is used
for more accurate battery capacity measurements with lower voltages.

This was unfortunately never documented in bindings, do so now.

The only in-tree user (sc2731.dtsi) has had interrupts specified
since its initial fuel-gauge submission and the current kernel
driver returns an error when no interrupt is specified, so also
add it to the required list.

Signed-off-by: Stanislav Jakubek <stano.jakubek@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/Zr3SAHlq5A78QvrW@standask-GA-A55M-S2HP


Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>