Add the MMC/SD driver for Allwinner SoCs.

This is based on the patch sent by Alexander Fedorov with the following
fixes/improvements:

• Better error handling;
• Clock is derived from PLL6 (obtained from netbsd);
• No more unnecessary busy loops on interrupt handler;
• style(9) fixes and code cleanup.

I also want to thanks Martin Galvan who has sent an alternative
implementation with some interesting fixes.

Tested on CubieBoard2, Banana-Pi (thanks to netgate!) and Cubieboard1
(Pratik Singhal).

This is intended to pave the way for the upcoming GSoC work (and make
easier the build of images for the supported boards).

PR: 196081
Submitted by: Alexander Fedorov <alexander.fedorov@rtlservice.com>