staging: rts5208: add support for rts5208 and rts5288

There are still many rts5208/5288 card readers being used, but no
drivers are supported them in kernel now. This driver can make a
great convenience for people who use them.

Many other rts-series card reader are supported by mfd driver, but due
to much difference with others, rts5208/5288 can not add into mfd driver
pretty now, so we provide a separated driver here to support the device.

Signed-off-by: Micky Ching <micky_ching@realsil.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>