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>