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>

scripts/Makefile.lib

@@ -329,7 +329,7 @@ cmd_acpi_c_asl=         \
 	iasl -p $< -tc -va $<.tmp; \
 	mv $(patsubst %.asl,%.hex,$<) $@
 
-$(obj)/%.c:    $(src)/%.asl
+$(obj)/dsdt.c:    $(src)/dsdt.asl
 	$(call cmd,acpi_c_asl)
 
 # Bzip2