The clock index "CLK_APMIXED_MFGPLL" belongs to the "apmixedsys" provider,
so fix the index.
In addition, add a "mfg1" label so following commits could set
domain-supply for MFG1 power domain.
Fixes: b8369604050b ("UPSTREAM: arm64: dts: mediatek: mt8188: Add support for SoC power domains")
Signed-off-by: Pablo Sun <pablo.sun@xxxxxxxxxxxx>