Add constant_current_range selector for LED driving

The AW9523 supports 256-step linear constant-current dimming when LED driving.
As noted in the datasheet introduction, the maximum current can be configured:

https://cdn-shop.adafruit.com/product-files/4886/AW9523+English+Datasheet.pdf

> The default maximum current (IMAX) is 37mA, and IMAX can be changed in GCR
> register.

In Table 10 ("GCR, Global control register (11H)"), the datasheet explains that
4 possible current ranges can be selected - maintaining the 256-step
granularity over a shorter current range:

> D[1:0] is used to configure the max drive current of LED. AW9523B set max
> current IMAX to 37mA(typical) default, and through register ISEL[1:0] can
> set to IMAX×1/4,IMAX×2/4, IMAX×3/4,IMAX, so the 256 step dimming range changes.

This change updates the `adafruit_aw9523.AW9523` class to have a
`constant_current_range` selector field that can be used to set the register
value. The 4 options are provided as constants (`AW9523_4_4_RANGE`,
`AW9523_3_4_RANGE`, etc).

An example of using this code would be:

```
aw = adafruit_aw9523.AW9523(i2c)
aw.constant_current_range = adafruit_aw9523.AW9523_3_4_RANGE
```

Author: rtyley

adafruit_aw9523.py

@@ -29,6 +29,7 @@
 from adafruit_bus_device import i2c_device
 from adafruit_register.i2c_struct import ROUnaryStruct, UnaryStruct
 from adafruit_register.i2c_bit import RWBit
+from adafruit_register.i2c_bits import RWBits
 from micropython import const
 
 __version__ = "0.0.0-auto.0"
@@ -46,6 +47,12 @@
 
 # pylint: disable=invalid-name
 
+# constant_current_range options
+AW9523_4_4_RANGE = const(0)  # Full range: 0 - 37   mA (typical)
+AW9523_3_4_RANGE = const(1)  # 3/4  range: 0 - 27.8 mA
+AW9523_2_4_RANGE = const(2)  # 2/4  range: 0 - 18.5 mA
+AW9523_1_4_RANGE = const(3)  # 1/4  range: 0 -  9.3 mA
+
 
 class AW9523:
     """CircuitPython helper class for using the AW9523 GPIO expander"""
@@ -67,6 +74,9 @@ class AW9523:
     # Whether port 0 is push-pull
     port0_push_pull = RWBit(_AW9523_REG_GCR, 4)
 
+    # 256-step constant-current range selector 'ISEL' - choice of 'full' (4/4), 3/4, 2/4, or 1/4.
+    constant_current_range = RWBits(2, _AW9523_REG_GCR, 0)
+
     def __init__(self, i2c_bus, address=_AW9523_DEFAULT_ADDR, reset=True):
         self.i2c_device = i2c_device.I2CDevice(i2c_bus, address)
         self._buffer = bytearray(2)