From Cortex-M reference manuals, the nvic supports up to 240 interrupts.
So the number of entries in vectors table is up to 256.

This patch adds a new config flag to specify the number of external interrupts.
Some ifdeferies are added in order to respect the natural alignment without
wasting too much space on smaller systems.
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Stefan Agner <stefan@agner.ch>
Tested-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>