mfd: tps65086: Make interrupt line optional

The BeagleV Starlight v0.9 board[1] doesn't have the IRQB line routed to
the SoC, but it is still useful to be able to reach the PMIC over I2C
for the other functionality it provides such as GPIOs and regulator
settings.

[1] https://github.com/beagleboard/beaglev-starlight



Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>