mfd: Add realtek USB card reader driver

Realtek USB card reader provides a channel to transfer command or data to flash
memory cards. This driver exports host instances for mmc and memstick subsystems
and handles basic works.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Roger Tseng <rogerable@realtek.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>