overlays: Rework sdio overlays to allow polling

MMC/SD interfaces support a DT property (non-removable) that disables
the usual polling required to detect card removal for a configuration
that doesn't have hardware card presence detection. This property is
required for the on-board SDIO WiFi interface, but other uses of the
interface may want the polling to be re-enabled.

'non-removable' is a boolean DT property - true if present, false if
absent - and the overlay mechanism does not allow a property in the
base DTB to be deleted, so if the base DTB has non-removable set
(which is true for all WiFi-equipped Pis) then an overlay cannot
unset it.

Modify the SDIO overlays to work around this problem by disabling
the mmc node and adding a clone to which non-removable may
optionally be added.

N.B. The default state of poll_once is still true, and the overlay
does include a non-removable property, but setting poll_once to false
("off") will remove the property from the overlay before it is applied.

See: https://github.com/raspberrypi/linux/issues/2401



Signed-off-by: Phil Elwell <phil@raspberrypi.org>