[Bug Report] elementui datetimepicker 想要达到timepicker相同的固定时间点效果

### Element UI version
2.15.10

### OS/Browsers version
无

### Vue version
2.7.13

### Reproduction Link
https://github.com/ElemeFE/element/issues/new

### Steps to reproduce
不需重现，:picker-options="{
start: '08:30',
step: '00:15',
end: '18:30'
}"设置:picker-options没有用

### What is Expected?
datetimepicker选择时间时也可以使用timepicker固定时间点效果

### What is actually happening?
实际行为是不可以去设置的