Fix the following two build warnings in function 'write_acpi_tables':

  warning: format '%lx' expects argument of type 'long unsigned int',
  but argument 2 has type 'u32' [-Wformat=]

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>