.gitignore: add SPDX License Identifier

Add SPDX License Identifier to all .gitignore files.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 #
 # NOTE! Don't add files that are generated in specific
 # subdirectories here. Add them in the ".gitignore" file

Documentation/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 output
 *.pyc

Documentation/devicetree/bindings/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 *.example.dts
 processed-schema.yaml

Documentation/vm/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 page-types
 slabinfo

arch/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 i386
 x86_64

arch/alpha/kernel/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 vmlinux.lds

arch/arc/boot/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 uImage

arch/arc/kernel/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 vmlinux.lds

arch/arm/boot/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 Image
 zImage
 xipImage

arch/arm/boot/compressed/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 ashldi3.S
 bswapsdi2.S
 font.c

arch/arm/crypto/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 aesbs-core.S
 sha256-core.S
 sha512-core.S

arch/arm/kernel/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 vmlinux.lds

arch/arm/mach-at91/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 pm_data-offsets.h

arch/arm/mach-omap2/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 pm-asm-offsets.h

arch/arm/vdso/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 vdso.lds
 vdso.so.raw
 vdsomunge

arch/arm64/boot/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 Image
 Image.gz

arch/arm64/crypto/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 sha256-core.S
 sha512-core.S

arch/arm64/kernel/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 vmlinux.lds

arch/arm64/kernel/vdso/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 vdso.lds

arch/arm64/kernel/vdso32/.gitignore

+# SPDX-License-Identifier: GPL-2.0-only
 vdso.lds
 vdso.so.raw