imx: spl: Partially revert "spl: eMMC/SD: Provide one __weak spl_boot_mode() function"

Commit d695d662 ("spl: eMMC/SD: Provide one __weak spl_boot_mode()
function") breaks the boot on several i.MX6 boards,
such as cuboxi and wandboard:

U-Boot SPL 2018.03-rc1-00212-g48914fc119 (Feb 10 2018 - 11:04:33 +1300)
Trying to boot from MMC1
Failed to mount ext2 filesystem...
spl_load_image_ext: ext4fs mount err - 0

Partially revert it so that we can boot U-Boot again on these
i.MX6 platforms.
Reported-by: NJonathan Gray <jsg@jsg.id.au>
Signed-off-by: NFabio Estevam <fabio.estevam@nxp.com>
Tested-by: NBreno Lima <breno.lima@nxp.com>

arch/arm/mach-imx/spl.c

@@ -132,6 +132,29 @@ int g_dnl_bind_fixup(struct usb_device_descriptor *dev, const char *name)
 }
 #endif
 
+#if defined(CONFIG_SPL_MMC_SUPPORT)
+/* called from spl_mmc to see type of boot mode for storage (RAW or FAT) */
+u32 spl_boot_mode(const u32 boot_device)
+{
+	switch (spl_boot_device()) {
+	/* for MMC return either RAW or FAT mode */
+	case BOOT_DEVICE_MMC1:
+	case BOOT_DEVICE_MMC2:
+#if defined(CONFIG_SPL_FAT_SUPPORT)
+		return MMCSD_MODE_FS;
+#elif defined(CONFIG_SUPPORT_EMMC_BOOT)
+		return MMCSD_MODE_EMMCBOOT;
+#else
+		return MMCSD_MODE_RAW;
+#endif
+		break;
+	default:
+		puts("spl: ERROR:  unsupported device\n");
+		hang();
+	}
+}
+#endif
+
 #if defined(CONFIG_SECURE_BOOT)
 
 /*