Add support for customized parent indices for MediaTek muxes: this is
necessary for the case in which we want to exclude some clocks from
a mux's parent clocks list, where the exclusions are not from the
very bottom of the list but either in the middle or the beginning.
Example:
- MUX1 (all parents)
- parent1; idx=0
- parent2; idx=1
- parent3; idx=2
- MUX1 (wanted parents)
- parent1; idx=0
- parent3; idx=2
To achieve that add a `parent_index` array pointer to struct mtk_mux,
then in .set_parent(), .get_parent() callbacks check if this array
was populated and eventually get the index from that.
Also, to avoid updating all clock drivers for all SoCs, rename the
"main" macro to __GATE_CLR_SET_UPD_FLAGS (so, `__` was added) and
add the new member to it; furthermore, GATE_CLK_SET_UPD_FLAGS has
been reintroduced as being fully compatible with the older version.
The new parent_index can be specified with the new `_INDEXED`
variants of the MUX_GATE_CLR_SET_UPD_xxxx macros.