dt-bindings: power: supply: maxim,max8903: add DC and USB input current-limit controls

Add three optional properties to the MAX8903 charger binding to
describe board-level GPIO control of the DC and USB input current
limits:

DC input (TA / DOK pin):
  - dc-current-limit-gpios (1..4 GPIOs): mux control lines feeding
    the MAX8903 IDC resistor mux;
  - dc-current-limit-mapping (uint32-matrix of {microamps,
    gpio_bit_pattern} pairs): the available current levels and the
    GPIO bit pattern that selects each level.

USB input (USB / UOK pin):
  - usb-current-limit-gpios: a single GPIO driving the IUSB pin.
    The IUSB pin is silicon-fixed by the MAX8903 datasheet (Pin
    Description): low selects 100 mA, high selects 500 mA. Only
    the GPIO mapping is board-specific; the current values are
    not exposed in DT.

A dependentRequired block ties dc-current-limit-gpios and
dc-current-limit-mapping together so an incomplete DT cannot pass
validation with only one half of the pair.

These let userspace clamp the input draw via the standard
POWER_SUPPLY_PROP_INPUT_CURRENT_LIMIT power_supply attribute. The HP
TouchPad uses both: two TLMM lines select between 0.5 A, 1.0 A,
1.5 A and 2.0 A DC input current limits behind the MAX8903B
charger, and a third TLMM line picks the IUSB 100 mA / 500 mA
limit.

These are purely additive; existing platforms remain unaffected.

Signed-off-by: Herman van Hazendonk <github.com@herrie.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20260605-submit-power-max8903-dc-limit-v2-1-0c5396e98f14@herrie.org
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

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