powerpc: Replace GPL 2.0+ README.legal boilerplate with SPDX

Upstream Linux never had a "README.legal" file, but it was present
in early source releases of Linux/m68k.  It contained a simple copyright
notice and a link to a version of the "COPYING" file that predated the
addition of the "only valid GPL version is v2" clause.

Get rid of the references to non-existent files by replacing the
boilerplate with SPDX license identifiers.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/d91725ff1ed5d4b6ba42474e2ebfeebe711cba23.1695031668.git.geert@linux-m68k.org