arm: caches: protect dram_bank_mmu_setup access to bi_dram

Add protection in dram_bank_mmu_setup() to avoid access to bd->bi_dram
before relocation.

This patch allow to use the generic weak function dram_bank_mmu_setup
to activate the MMU and the data cache in SPL or in U-Boot before
relocation, when bd->bi_dram is not yet initialized.

In this cases, the MMU must be initialized explicitly with
mmu_set_region_dcache_behaviour function.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/arch/arm/lib/cache-cp15.c b/arch/arm/lib/cache-cp15.c
index f8d2096..54509f1 100644
--- a/arch/arm/lib/cache-cp15.c
+++ b/arch/arm/lib/cache-cp15.c
@@ -91,6 +91,10 @@
 	bd_t *bd = gd->bd;
 	int	i;
 
+	/* bd->bi_dram is available only after relocation */
+	if ((gd->flags & GD_FLG_RELOC) == 0)
+		return;
+
 	debug("%s: bank: %d\n", __func__, bank);
 	for (i = bd->bi_dram[bank].start >> MMU_SECTION_SHIFT;
 	     i < (bd->bi_dram[bank].start >> MMU_SECTION_SHIFT) +