[Bug Report] missing "maxlength" default props in Element.Input

### Element UI version
2.15.2

### OS/Browsers version
windows chrome

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/ziyoung/pen/rRKYpd?editors=1011

### Steps to reproduce
some props could not set global default value. eg `maxlength`
https://element.eleme.io/#/zh-CN/component/input

> Element.Input.props
```
{value: Array(2), size: ƒ, resize: ƒ, form: ƒ, disabled: ƒ, …}
autoComplete: {type: ƒ, validator: ƒ}
autocomplete: {default: 'off', type: ƒ}
autosize: {type: Array(2), default: false}
clearable: {default: false, type: ƒ}
disabled: ƒ Boolean()
form: ƒ String()
label: ƒ String()
prefixIcon: ƒ String()
readonly: ƒ Boolean()
resize: ƒ String()
showPassword: {default: false, type: ƒ}
showWordLimit: {default: false, type: ƒ}
size: ƒ String()
suffixIcon: ƒ String()
tabindex: ƒ String()
type: {default: 'text', type: ƒ}
validateEvent: {default: true, type: ƒ}
value: (2) [ƒ, ƒ]
[[Prototype]]: Object
```
### What is Expected?
able to set global input maxlength
`Element.Input.props.maxlength.default = 10;`

### What is actually happening?
unable to set global input maxlength