hw/arm/stellaris: Add 'armv7m' local variable

hw/arm/stellaris: Add 'armv7m' local variable

While the TYPE_ARMV7M object forward its NVIC interrupt lines,
it is somehow misleading to name it 'nvic'. Add the 'armv7m'
local variable for clarity, but also keep the 'nvic' variable
behaving like before when used for wiring IRQ lines.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20250112225614.33723-3-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>