.gitignore: drop *.dts.tmp pattern

This pattern was added by commit cc4f427b to ignore the intermidiate
file for generating DTB.

When Kbuild was introduced, dts/Makefile was totally re-written.
This ignore pattern is already useless.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

.gitignore

@@ -20,7 +20,6 @@
 *.bin
 *.patch
 *.cfgtmp
-*.dts.tmp
 
 # Build tree
 /build-*