MFC r349638, r350161, r351186

r349638 by ganbold:
Subclass Rockchip's General Register Files driver from Simple MFD driver.

r350161 by ganbold:
Add driver for Rockchip RK3399 eMMC PHY.
Tested on NanoPC-T4 board.

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D20840

r351186 by mmel:
Improve rk_i2c driver:

• Properly handle IIC_M_NOSTOP and IIC_M_NOSTART flags.
• add polling mode, so driver can be used even if interrupts are not enabled (this is necessary for proper support of PMICs).
• add support for RK3288