gpio_deep_sleep_hold_en() function

Enable all digital gpio pads hold function during Deep-sleep. Enabling this feature makes all digital gpio pads be at the holding state during Deep-sleep. The state of each pad holds is its active configuration (not pad's sleep configuration!). Note: 1. For digital IO, this API takes effect only if the corresponding digital IO pad hold function has been enabled. You can enable the GPIO pad hold function by calling `gpio_hold_en`. has been enabled. You can call `gpio_hold_en` to enable the gpio pad hold function. 2. Though this API targets all digital IOs, the pad hold feature only works when the chip is in Deep-sleep mode. When the chip is in active mode, the digital GPIO state can be changed freely even if you have called this function, except for IOs that are already held by `gpio_hold_en`. After this API is being called, the digital gpio Deep-sleep hold feature will work during every sleep process. You should call `gpio_deep_sleep_hold_dis` to disable this feature.