x86: baytrail: Add internal UART ASL description

x86: baytrail: Add internal UART ASL description

BayTrail integrates an internal ns15550 compatible UART (PNP0501).
Its IRQ is hardwired to IRQ3 in old revision chipset, but in newer
revision one IRQ4 is being used for ISA compatibility. Handle this
correctly in the ASL file.

Linux does not need this ASL, but Windows need this to correctly
discover a COM port existing in the system so that Windows can
show it in the 'Device Manager' window, and expose this COM port
to any terminal emulation application.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>