spl: mmc: Rename spl_boot_partition() to spl_mmc_boot_partition()

This function is only relevant to the MMC driver so calling it
spl_boot_partition() might be confusing.  Rename it to
spl_mmc_boot_partition() to make its purpose more clear (and bring
it in line with spl_mmc_boot_mode()).

Signed-off-by: Harald Seiler <hws@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>