spl: mmc: Rename spl_boot_mode() to spl_mmc_boot_mode()

The function's name is misleading as one might think it is used
generally to select the boot-mode when in reality it is only used by the
MMC driver to find out in what way it should try reading U-Boot Proper
from a device (either using a filesystem, a raw sector/partition, or an
eMMC boot partition).

Rename it to spl_mmc_boot_mode() to make it more obvious what this
function is about.

Link: https://lists.denx.de/pipermail/u-boot/2020-April/405979.html
Signed-off-by: Harald Seiler <hws@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/mach-k3/j721e_init.c b/arch/arm/mach-k3/j721e_init.c
index f34090f..71fc20c 100644
--- a/arch/arm/mach-k3/j721e_init.c
+++ b/arch/arm/mach-k3/j721e_init.c
@@ -223,7 +223,7 @@
 #endif
 }
 
-u32 spl_boot_mode(const u32 boot_device)
+u32 spl_mmc_boot_mode(const u32 boot_device)
 {
 	switch (boot_device) {
 	case BOOT_DEVICE_MMC1: