x86: Provide weak PC/AT compatibility setup function

It is possibly to setup x86 boards to use non-PC/AT configurations. For
example, the sc520 is an x86 CPU with PC/AT and non-PC/AT peripherals.
This function allows the board to set itself up for maximum PC/AT
compatibility just before booting the Linux kernel (the Linux kernel
'just works' if everything is PC/AT compliant)

Signed-off-by: Graeme Russ <graeme.russ@gmail.com>