ARM: configs: stm32: Set CPU_V7M_NUM_IRQ to max value

ARM: configs: stm32: Set CPU_V7M_NUM_IRQ to max value

stm32_defconfig is used for several STM32 MCU: STM32F429, STM32F469,
STM32F746 and now STM32H743. Each of MCU listed have different interrupts
number mapped on NVIC. STM32F429: 81, STM32F469: 92, STM32F746: 97 and
STM32H743: 149. I could set CPU_V7M_NUM_IRQ to 149 but in order to avoid
forgetting to update this value for next STM32 MCU I prefer to set it to
max value: 240.

Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>