ARM: mmu: Introduce weak dram_bank_setup function

Introduce a weak version of dram_bank_setup function
to allow a platform specific function.

This is used in the subsequent patch to setup dram region
without 'XN' attribute in order to enable the region
under client permissions.

Signed-off-by: R Sricharan <r.sricharan@ti.com>
Cc: Vincent Stehle <v-stehle@ti.com>
Cc: Tom Rini <trini@ti.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>