arch/x86/acpigen: Add generator for _ROM

arch/x86/acpigen: Add generator for _ROM

Generate ACPI AML code for _ROM method.
This function takes as input ROM data and ROM length.

 Arguments passed into _DSM method:
 Arg0 = Offset in Bytes
 Arg1 = Bytes to return

Example:
 acpigen_write_rom(0xdeadbeef, 0x10000)

 AML code generated would look like:
        Method (_ROM, 2, NotSerialized)  // _ROM: Read-Only Memory
        {
            OperationRegion (ROMS, SystemMemory, 0xdeadbeef, 0x00010000)
            Field (ROMS, AnyAcc, NoLock, Preserve)
            {
                Offset (0x00),
                RBF0,   524288
            }

            Local0 = Arg0
            Local1 = Arg1
            If (Local1 > 0x1000)
            {
                Local1 = 0x1000
            }

            If (Local0 > 0x00010000)
            {
                Return (Buffer (Local1)
                {
                     0x00
                })
            }

            If (Local0 > 0x0f000)
            {
                Local2 = 0x10000 - Local0
                If (Local1 > Local2)
                {
                     Local1 = Local2
                }
            }

            Name (ROM1, Buffer (Local1)
            {
                 0x00
            })
            Local1 *= 0x08
            Local0 *= 0x08
            CreateField (RBF0, Local0, Local1, TMPB)
            ROM1 = TMPB /* \_SB_.PCI0.GFX0._ROM.TMPB */
            Return (ROM1) /* \_SB_.PCI0.GFX0._ROM.ROM1 */
        }

Change-Id: Ie118b15257295b7133c8e585c0fd5218249dec8d
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/20547
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>