mmc: rtsx: Add test mode for RTS5261

This patch add test mode for RTS5261.
If test mode is set, reader will switch to SD Express mode
mandatorily, and this mode is used by factory testing only.

Signed-off-by: Rui Feng <rui_feng@realsil.com.cn>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/1604397269-2780-1-git-send-email-rui_feng@realsil.com.cn


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>