armv7-m: Return DeviceState* from armv7m_init()

Change armv7m_init to return the DeviceState* for the NVIC.
This allows access to all GPIO blocks, not just the IRQ inputs.
Move qdev_get_gpio_in() calls out of armv7m_init() into
board code for stellaris and stm32f205 boards.

Signed-off-by: Michael Davidsaver <mdavidsaver@gmail.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>