drivers: charger: charger_max20355: fix configuration

Maximum charging current is selected with an external resistor in
the MAX20335 charger. Therefore it is not possible to configure it
with software directly. There is only a capability to limit current
set with hardware but configuration of the limiter is not
straight-forward.

To reflect real functionality, drop usage of
`constant-charge-current-max-microamp` property as an required one and
use custom `chgin-to-sys-current-limit-microamp` instead.
Use enum in binding file to signalize improper values at compile time.

Drop support for `CHARGER_PROP_CONSTANT_CHARGE_CURRENT_UA` API property
since this cannot be handled.

The `max20335_get_constant_charge_current()` function become useless so
remove it.

Signed-off-by: Lukasz Madej <l.madej@grinn-global.com>