dt-bindings: at24: add the 'num-addresses' property

Currently the at24 driver only creates additional i2c dummies for
atmel,24c00 and it's hard-coded. Some other chips (like for example
Microchip's 24AA02T) also take more slave addresses despite being
otherwise compatible with already supported variants.

Add a new property to the device tree binding document that defines
the total number of i2c slave addresses taken by the device. The
addresses are counted starting from the one in the reg property.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>