mmc: rtsx_pci: Add SD Express mode support for RTS5261

mmc: rtsx_pci: Add SD Express mode support for RTS5261

RTS5261 support SD mode and PCIe/NVMe mode. The workflow is as follows.
1.RTS5261 work in SD mode and set MMC_CAPS2_SD_EXP flag.
2.If card is plugged in, Host send CMD8 to ask card's PCIe availability.
3.If the card has PCIe availability and WP is not set, init_sd_express() will be invoked,
RTS5261 switch to PCIe/NVMe mode.
4.Mmc driver handover it to NVMe driver.
5.If card is unplugged, RTS5261 will switch to SD mode.

Signed-off-by: Rui Feng <rui_feng@realsil.com.cn>
Link: https://lore.kernel.org/r/1603936703-3403-1-git-send-email-rui_feng@realsil.com.cn
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>