lib: fdtdec: Update ram_base to store ram start adddress

This patch updates the ram_base to store the start address of
the first bank DRAM and the use this ram_base to calculate ram_top
properly. This patch fixes the erroneous calculation of ram_top
incase of non zero ram start address.

Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@konsulko.com>