riscv: dts: microchip: update pic64gx gpio interrupts to better match the SoC

riscv: dts: microchip: update pic64gx gpio interrupts to better match the SoC

Just like PolarFire SoC, the same issues with GPIO interrupts exist in
the pic64gx, due to their similarity. Yoinking from the commit message
for the same change for PolarFire SoC:

There are 3 GPIO controllers on this SoC, of which:
- GPIO controller 0 has 14 GPIOs
- GPIO controller 1 has 24 GPIOs
- GPIO controller 2 has 32 GPIOs

All GPIOs are capable of generating interrupts, for a total of 70.
There are only 41 IRQs available however, so a configurable mux is used
to ensure all GPIOs can be used for interrupt generation.
38 of the 41 interrupts are in what the documentation calls "direct
mode", as they provide an exclusive connection from a GPIO to the PLIC.
The 3 remaining interrupts are used to mux the interrupts which do not
have a exclusive connection, one for each GPIO controller.

The mux was overlooked when the bindings and driver were originally
written for the GPIO controllers on Polarfire SoC, and the interrupts
property in the GPIO nodes used to try and convey what the mapping was.
Instead, the mux should be a device in its own right, and the GPIO
controllers should be connected to it, rather than to the PLIC.
Now that a binding exists for that mux, fix the inaccurate description
of the interrupt controller hierarchy.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>