acpi: Change build log for ASL files

Currently when compiling U-Boot with ASL file, the build log says:

  ASL     board/intel/bayleybay/dsdt.c

This looks odd as ASL compiler's input is ASL file, not C file.
Change the make rule to use $< instead.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>