Add functions for USBPD

[21km43]

• Add functions for usbpd.

uint8_t usbpd_sink_get_pdo_num(void);
uint8_t usbpd_sink_get_pps_num(void);
uint16_t usbpd_sink_get_pdo_voltage(int index);
uint16_t usbpd_sink_get_pdo_current(int index);
uint16_t usbpd_sink_get_pps_min_voltage(int index);
uint16_t usbpd_sink_get_pps_max_voltage(int index);
uint16_t usbpd_sink_get_pps_current(int index);

[tako0910]

Sounds good. I am making the same changes in my personal repository.
Without these functions, it is not possible to retrieve PDO/PPS-related values.
Therefore, this pull request is useful.

(In #137, I mentioned that I would create a separate PR, but I didn’t.
Thank you for publishing instead!)