staging: greybus: remove .gitignore file

Only the tools subdirectory needs a .gitignore entry, so move it there
and fix it up to only list the needed file.
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/.gitignore

-.*
-*.cmd
-*.ko
-*.mod.c
-modules.order
-Module.symvers
-*.o
-*.o.*
-*.swp
-.tmp_versions
-tags
-cscope.*
-ncscope.*
-*.patch
-tools/loopback_test

drivers/staging/greybus/tools/.gitignore

+loopback_test