dt-bindings: mfd: twl: Add missing sub-nodes for TWL4030 & TWL603x

Update the main TI TWL-family binding to be self-contained and to fix
pre-existing validation errors.

To ensure future patches are bisectable, child nodes whose bindings
are in other patches (audio, keypad, usb, etc.) are now defined using
a flexible 'additionalProperties: true' pattern. This removes hard
dependencies between the MFD and subsystem bindings.

The complete dtbs_check for this binding is clean except for two
warnings originating from pre-existing bugs in the OMAP DTS files,
for which fixes have already been submitted separately [1][2].

Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Uwe Kleine-König <ukleinek@kernel.org>
Signed-off-by: Jihed Chaibi <jihed.chaibi.dev@gmail.com>
Reviewed-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Lee Jones <lee@kernel.org>