[E81][DTS] Add uSD slot on SDC2 interface (sdhc_2)

- Increase drive-strength to 10 and add bias-pull-up as per Qualcomm suggestion
- Let vreg_l6c_2v96 voltage switch between 3.3V and 1.8
- Add reg_3v3_sdcard as card power supply
- Use GPIO_91 as card detect