SPI plugin: add support for irq based handling of handshake bits

Add support for irq based handling of handshake bits. This reduces CPU load since in polling the handshake flags is done via busy wait.

NOTE: This mode uses linux GPIO framework which does not support level triggered interrupts as required by netX devices. A workaround has been introduced in the ISR, which should prevent missing an interrupt. It has been verified to work under default conditions, but may not always work.