acpi: Explicitly spell out dsdt.c in the make rule

acpi: Explicitly spell out dsdt.c in the make rule

Currently the make rule for dsdt.c uses a wildcard, as below:

  $(obj)/%.c:    $(src)/%.asl

To avoid any side effect, explicitly mention dsdt.c as this is
the file we intend to use for ACPI DSDT AML generation.

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>