Enable A20 using KBC for some MSI laptops to fix S3 resume

Some MSI laptop BIOSes are broken - INT 15h code uses port 92h to enable A20 line but resume code assumes that KBC was used.
The laptop will not resume from S3 otherwise but powers off after a while and then powers on again stuck with a blank screen.

Fix it by enabling A20 using KBC. Affected laptop list and DMI data are from bug reports at Ubuntu Launchpad.

Also add kernel parameter to easily activate this quirk on any system.

Only compile tested. The original patch was tested with EX600 and PR200.

Fixes https://bugzilla.kernel.org/show_bug.cgi?id=12878

---
 Documentation/kernel-parameters.txt |    6 ++
 arch/x86/kernel/acpi/boot.c         |  122 +++++++++++++++++++++++++++++++++++
 2 files changed, 128 insertions(+), 0 deletions(-)

index 9776f06..eb3d6a4 100644
@@ -153,6 +153,12 @@ multipliers 'Kilo', 'Mega', and 'Giga', equalling 2^10, 2^20, and 2^30
 bytes respectively. Such letter suffixes can also be entirely omitted.
 
 
+	a20_enable_kbc	[ACPI,X86]
+			Enable A20 line using KBC even if it's already
+			enabled using other method.
+			This is needed for some MSI laptops to resume from
+			S3.
+
 	acpi=		[HW,ACPI,X86]
 			Advanced Configuration and Power Interface
 			Format: { force | off | strict | noirq | rsdt }

Ondrej Zary