hw/sh4/sh7750_regs: Replace link to license by its full content

hw/sh4/sh7750_regs: Replace link to license by its full content

This file is borrowed from the RTEMS source code, which comes
with a GPL-2.0-or-later license with a header exception.

Expand the GPL-2.0-or-later license in place to not be dependent
on a 3rd party website. This also fix the misleading comment "The
license and distribution terms for this file may be found in the
file LICENSE in this distribution" referring to the RTEMS distribution
and not to the QEMU one.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20210222185605.2714192-1-f4bug@amsat.org>