ARM: orion: use SPARSE_IRQ everywhere

As a preparation for multiplatform support, this moves all the
code using plat-orion over to use sparse irq support, which is
enabled implicitly for multiplatform.

In particular, the hardcoded NR_IRQS macro gets replaced with
a machine specific one that is set in the machine descriptor
in order to set up a static mapping for all legacy interrupts.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>