omap_hwmod: use a terminator record with omap_hwmod_mpu_irqs arrays

Previously, struct omap_hwmod_mpu_irqs arrays were unterminated; and
users of these arrays used the ARRAY_SIZE() macro to determine the
length of the array.  However, ARRAY_SIZE() only works when the array
is in the same scope as the macro user.

So far this hasn't been a problem.  However, to reduce duplicated
data, a subsequent patch will move common data to a separate, shared
file.  When this is done, ARRAY_SIZE() will no longer be usable.

This patch removes ARRAY_SIZE() usage for struct omap_hwmod_mpu_irqs
arrays and uses a sentinel value (irq == -1) as the array terminator
instead.

Signed-off-by: Paul Walmsley <paul@pwsan.com>