Implement proper RSDP validation in acpi_ut_read_table(). Prevents a segmentation
fault that can occur if a user passes the wrong file to iasl.

This patch is only useful for iasl, which is not shipped in the Linux
kernel.

After the new table reading utility functions are well tested, acpidump can
also switch to use the generic acpi_ut_read_table_xxx() APIs. Currently
this patch is no-op as acpidump does not link to the new APIs.
Signed-off-by: David E. Box <david.e.box@linux.intel.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>