extcon: axp288: Remove dependency on non-existing platform_data

When the extcon_axp288 driver was originally merged, it was merged with
a dependency on some other driver providing platform data for it.

However such another driver was never merged, so the extcon_axp288 as
merged upstream has never worked, its probe method simply always returns
-ENODEV.

This commit drops the dependency on the pdata always being there, instead
it treats not having pdata as the pdata having a NULL gpio_mux_control,
something which the code was already prepared to handle.

Note that the code for controlling the mux_control gpio is left in place,
as this may be necessary to allow the axp288 pmic to properly detect the
charger type (instead of assuming 500mA max charge current) on some
tablets. This will make it easier for future patches to add support for
this gpio by getting the gpio info from somewhere.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>