ppc: mpc8xx: Drop -mstring from PLATFORM_CPPFLAGS
This has not been supported by toolchains for some time and has been
putting out a warning. Drop this.
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>