armv7m: Use QOMified armv7m object in armv7m_init()

Make the legacy armv7m_init() function use the newly QOMified
armv7m object rather than doing everything by hand.

We can return the armv7m object rather than the NVIC from
armv7m_init() because its interface to the rest of the
board (GPIOs, etc) is identical.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1487604965-23220-5-git-send-email-peter.maydell@linaro.org