mfd: axp20x: Set explicit ID for AXP313 regulator

On newer boards featuring the A523 SoC, the AXP323 (related to the
AXP313) is paired with the AXP717 and serves as a secondary PMIC
providing additional regulator outputs. However the MFD cells are all
registered with PLATFORM_DEVID_NONE, which causes the regulator cells
to conflict with each other.

Commit e37ec3218870 ("mfd: axp20x: Allow multiple regulators") attempted
to fix this by switching to PLATFORM_DEVID_AUTO so that the device names
would all be different, however that broke IIO channel mapping, which is
also tied to the device names. As a result the change was later reverted.

Instead, here we attempt to make sure the AXP313/AXP323 regulator cell
does not conflict by explicitly giving it an ID number. This was
previously done for the AXP809+AXP806 pair used with the A80 SoC.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Link: https://lore.kernel.org/r/20250619173207.3367126-1-wens@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>