staging: mt7621-gpio: set different names for each gpio_chip and irq_chip

Currently the driver defines 3 gpiochips, one for each bank.

/sys/class/gpio/gpiochip416/label:1e000600.gpio
/sys/class/gpio/gpiochip448/label:1e000600.gpio
/sys/class/gpio/gpiochip480/label:1e000600.gpio

Unfortunately they all have the same label

Interrupts from /proc/interrupt show the same name which is
confusing:

/proc/interrupts:

17: 0  0  0  0  MIPS GIC  19  mt7621, mt7621, mt7621

which is the interrupt from the GPIO controller.
It is a little weird that all three banks are named "mt7621"
here. We also have:

26: 0  0  0  0  GPIO  18  reset

which is the interrupt from GPIO which provides the "reset"
button. I suspect that if I had interrupts form two different
banks they would both be called "GPIO" which would be a little
confusing.

In order to unify all of this set different names for each chip
Use a 'bank-based' name instead the same for all: 'mt7621-bank[0-2]'.
Create a new 'mediatek_gpio_bank_name' function which return the
name depending on the bank number. This function is allways called
with a valid index 0, 1 or 2.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>