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/am6_init.c b/arch/arm/mach-k3/am6_init.c
index 3768bcc..b692806 100644
--- a/arch/arm/mach-k3/am6_init.c
+++ b/arch/arm/mach-k3/am6_init.c
@@ -199,7 +199,7 @@
 #endif
 }
 
-u32 spl_boot_mode(const u32 boot_device)
+u32 spl_mmc_boot_mode(const u32 boot_device)
 {
 #if defined(CONFIG_SUPPORT_EMMC_BOOT)
 	u32 devstat = readl(CTRLMMR_MAIN_DEVSTAT);