arch: arm: mach-k3: am642_init: Correct the function name spl_boot_mode() to spl_mmc_boot_mode()

Function spl_boot_mode() is called in common/spl/spl_mmc.c, to find the
boot mode for a given boot device. This function was renamed to
spl_mmc_boot_mode() by commit e9759065.

Therefore, rename spl_boot_mode to spl_mmc_boot_mode.

Fixes: 57dba04a ("arm: mach-k3: am642: Add support for boot device detection")
Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210726152807.22991-2-a-govindraju@ti.com