1-wire bus platform device for sunxi platforms

This adds a way to register a platform device for 1-wire bus.
It introduces a new section "[w1_para]" in the FEX to configure the
GPIO pin number used for the bus, with the attribute named "gpio".
The GPIO pin must also be defined in the "[gpio_para]" section.

1-wire device can then be accessed using the w1-gpio driver.

Example configuration :
[w1_para]
gpio = 3

[gpio_para]
gpio_used = 1
gpio_num = 3
...
gpio_pin_3 = port:PH7<0><default><default><0>

It is also possible to configure the GPIO pin number with the
module paramerer "gpio". In this case, if the pin is valid,
it will be used instead of what is configured in the FEX script.

Signed-off-by: Damien Nicolet <zardam@gmail.com>