ARM: shmobile: r8a7779: Remove NR_IRQS_LEGACY

Remove NR_IRQS_LEGACY from the r8a7779 generic machine
vector. The generic r8a7779 machine vector requires use
of Multiplatform, and in such case SPARSE_IRQ is enabled
by default. This in turns means that the default value
of .nr_irqs equals NR_IRQS and NR_IRQS_LEGACY. Because
of this we can simply remove NR_IRQS_LEGACY and move one
step closer to a cruft-free environment.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>