Tables: Add mechanism to control early table checksum verification.

It is reported that Linux x86 kernel cannot map large tables. The following
large SSDT table on such platform fails to pass checksum verification and
cannot be installed:
 ACPI: SSDT 0x00000000B9638018 07A0C4 (v02 INTEL  S2600CP  00004000 INTL 20100331)

It sounds strange that in the 64-bit virtual memory address space, we
cannot map a single ACPI table to do checksum verification. The root cause
is:
1. ACPICA doesn't split IO memory mapping and table mapping;
2. Linux x86 OSL implements AcpiOsMapMemory() using a size limited fix-map
   mechanism during early boot stage, which is more suitable for only IO
   mappings.

ACPICA originally only mapped table header for signature validation, and
this header mapping is required by OSL override mechanism. There was no
checksum verification because we could not map the whole table using this
OSL. While the following ACPICA commit enforces checksum verification by
mapping the whole table during Linux boot stage and it finally triggers
this issue on some platforms:
 Commit: bf959821a98985b69b118ad6e6676a142c2e3241
 Subject: Tables: Fix table checksums verification before installation.

Before doing further cleanups for the OSL table mapping and override
implementation, this patch introduces an option for such OSPMs to
temporarily discard the checksum verification feature. It then can be
re-enabled easily when the ACPICA and the underlying OSL is ready.

This patch also deletes a comment around the limitation of mappings because
it is not correct. The limitation is not how many times we can map in the
early stage, but the OSL mapping facility may not be suitable for mapping
the ACPI tables and thus may complain us the size limitation.

The AcpiTbVerifyTable() is renamed to AcpiTbVerifyTempTable() due to the
work around added, it now only applies to the table descriptor that hasn't
been installed and cannot be used in other cases. Lv Zheng.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Reported-and-tested-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>