mmc: allow setting slot index via device tree alias

With GPIO, UART and others, allow specifying the device index via the
aliases node in the device tree.

On embedded devices, there is often a combination of removable (e.g.
SD card) and non-removable MMC devices (e.g. eMMC).
Therefore the index might change depending on

* host of removable device
* removable card present or not

This makes it difficult to hardcode the root device, if it is on the
non-removable device. E.g. if SD card is present eMMC will be mmcblk1,
if SD card is not present at boot, eMMC will be mmcblk0.

Alternative solutions like PARTUUIDs do not cover the case where multiple
mmcblk devices contain the same image. This is a common issue on devices
that can boot both from eMMC (for regular boot) and SD cards (as a
temporary boot medium for development). When a firmware image is
installed to eMMC after a test boot via SD card, there will be no
reliable way to refer to a specific device using (PART)UUIDs oder
LABELs.

The demand for this feature has led to multiple attempts to implement
it, dating back at least to 2012 (see
https://www.spinics.net/lists/linux-mmc/msg26586.html for a previous
discussion from 2014).

All indices defined in the aliases node will be reserved for use by the
respective MMC device, moving the indices of devices that don't have an
alias up into the non-reserved range. If the aliases node is not found,
the driver will act as before.

This is a rebased and cleaned up version of
https://www.spinics.net/lists/linux-mmc/msg26588.html .

Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/


Git-commit: fa2d0aa96941e8dc347e49a04c75468e3002ee20_
Change-Id: I88c11705c8b66022e67b38a64bbc91625357a4bc
Based-on-patch-by: Sascha Hauer <s.hauer@pengutronix.de>
Link: https://lkml.org/lkml/2020/8/5/194


Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Signed-off-by: Ram Prakash Gupta <rampraka@codeaurora.org>