dt-bindings: gpio: lpc18xx: describe interrupt controllers of GPIO controller

From LPC18xx and LPC43xx User Manuals the GPIO controller consists of
the following weakly connected blocks:
* GPIO pin interrupt block at 0x40087000,
* GPIO GROUP0 interrupt block at 0x40088000,
* GPIO GROUP1 interrupt block at 0x40089000,
* GPIO port block at 0x400F4000.

While all 4 sub-controller blocks have their own I/O addresses, moreover
all 3 interrupt blocks are APB0 peripherals and high-speed GPIO block is
an AHB slave, according to the hardware manual interrupt controllers and
GPIO controller block are seen as a single device, all 4 sub-controllers
have the shared reset signal RGU #28 and the same shared clock to access
registers CLK_Mx_GPIO on CCU1.

The change adds descriptions of the currently missing interrupt controller
blocks found on GPIO controller, new added properties are 'reg-names',
'resets', 'interrupt-controller' and '#interrupt-cells', also the example
is updated to reflect the changes in device tree binding description.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>