ARM: use ARM_SINGLE_ARMV7M for ARMv7-M platforms

Use the new config symbol ARM_SINGLE_ARMV7M which groups config
symbols used by modern ARMv7-M platforms. This allows supporting
multiple ARMv7-M platforms in one kernel image. However, a common
kernel image requires the combined platforms to share the same
main memory layout to be bootable.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Joachim Eastwood <manabian@gmail.com>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>