armv7m: Don't put core v7M devices under CONFIG_STELLARIS

The NVIC is a core v7M device that exists for all v7M CPUs;
put it under a CONFIG_ARM_V7M rather than hiding it under
CONFIG_STELLARIS.

(We'll use CONFIG_ARM_V7M for the SysTick device too
when we split it out of the NVIC.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1487604965-23220-9-git-send-email-peter.maydell@linaro.org