ARM64: zynqmp: Replace board specific with generic memory bank decoding

The dram_init and dram_init_banksize functions were using a board
specific implementation for decoding the memory banks from the fdt. This
board specific implementation uses a static variable 'tmp' which makes
these functions unsafe for execution from within the board_init_f
context.

This change makes the dram_init* functions use a generic implementation
of decoding and populating memory bank and size data.

Signed-off-by: Nathan Rossi <nathan@nathanrossi.com>
Fixes: 8d59d7f6 ("ARM64: zynqmp: Read RAM information from DT")
Cc: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>