spl: mmc: extend spl_mmc_boot_mode() to take mmc argument

Platforms can overwrite the weak definition of spl_mmc_boot_mode() to
determine where to load U-Boot proper from.
For most of them this is a trivial decision based on Kconfig variables,
but it might be desirable the probe the actual device to answer this
question.

Pass the pointer to the mmc struct to that function, so implementations
can make use of that.

Compile-tested for all users changed.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Ley Foon Tan <ley.foon.tan@inte.com> (for SoCFPGA)
Acked-by: Lokesh Vutla <lokeshvutla@ti.com> (for OMAP and K3)
Reviewed-by: Simon Glass <sjg@chromium.org>