stm32f205: Rename 'nvic' local to 'armv7m'

The local variable 'nvic' in stm32f205_soc_realize() no longer
holds a direct pointer to the NVIC device; it is a pointer to
the ARMv7M container object. Rename it 'armv7m' accordingly.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1487604965-23220-12-git-send-email-peter.maydell@linaro.org