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>
diff --git a/arch/x86/include/asm/acpi_table.h b/arch/x86/include/asm/acpi_table.h
index 8003850..d5d77cc 100644
--- a/arch/x86/include/asm/acpi_table.h
+++ b/arch/x86/include/asm/acpi_table.h
@@ -330,6 +330,15 @@
 ulong write_acpi_tables(ulong start);
 
 /**
+ * acpi_get_rsdp_addr() - get ACPI RSDP table address
+ *
+ * This routine returns the ACPI RSDP table address in the system memory.
+ *
+ * @return:	ACPI RSDP table address
+ */
+ulong acpi_get_rsdp_addr(void);
+
+/**
  * acpi_find_fadt() - find ACPI FADT table in the sytem memory
  *
  * This routine parses the ACPI table to locate the ACPI FADT table.