mmc: Add new VUB300 USB-to-SD/SDIO/MMC driver

Add a driver for Elan Digital System's VUB300 chip which is a USB
connected SDIO/SDmem/MMC host controller.  A VUB300 chip enables a USB 2.0
or USB 1.1 connected host computer to use SDIO/SD/MMC cards without the
need for a directly connected, for example via PCI, SDIO host controller.

Signed-off-by: Anthony F Olech <tony.olech@elandigitalsystems.com>
[cjb: various punctuation and style fixes]
Tested-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Chris Ball <cjb@laptop.org>