ARM: ep93xx: enable SPARSE_IRQ

Without CONFIG_SPARSE_IRQ, we rely on mach/irqs.h to define NR_IRQS
globally. Do the minimal conversion by setting .nr_irqs in each
machine descriptor.

Only the vision_ep9307 machine has extra IRQs for GPIOs, so make
.nr_irqs the original value there, while using the plain NR_EP93XX_IRQS
everywhere else.

Cc: Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Hubert Feurstein <hubert.feurstein@contec.at>
Cc: Lukasz Majewski <lukma@denx.de>
Tested-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>