uboot-imx

.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>