spl: eMMC/SD: Provide one __weak spl_boot_mode() function

The goal of this patch is to clean up the code related to choosing SPL
MMC boot mode.

The spl_boot_mode() now is called only in spl_mmc_load_image() function,
which is only compiled in if CONFIG_SPL_MMC_SUPPORT is enabled.

To achieve the goal, all per mach/arch implementations eligible for
unification has been replaced with one __weak implementation.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Acked-by: Michal Simek <michal.simek@xilinx.com> (For ZynqMP)
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>