uboot-imx

x86: acpi: Use an API to get the ACPI RSDP table address

At present the acpi_rsdp_addr variable is directly referenced in
setup_zimage(). This changes to use an API for better encapsulation
and extension.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>