SPDX-License-Identifier: fixing some problematic GPL-2.0 files

Unlike the other patches in this series so far, this commit fixes a
ambiguity in the license terms for some OMAP files:  the code was
originally derived from the Linux kernel sources, where it was clearly
marked as GPL-2.0 (i. e. without the "or later" part), but the U-Boot
version had a GPL-2.0+ file header added, apparently without
permission / relicensing from the original authors of the code.

Insert a GPL-2.0 SPDX-License-Identifier to fix this.

Signed-off-by: Wolfgang Denk <wd@denx.de>
cc: Tom Rix <Tom.Rix@windriver.com>
Cc: Tom Rini <trini@ti.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Acked-by: Tom Rini <trini@ti.com>