sh: Fix up simplified multi-evt handling under sparseirq.

This fixes up the simplified multi-evt handling when sparseirq support is
enabled. While vectors are redirected through the single unique masking
source, each one of the redirected vectors still requires its own backing
irq_desc, which needs to be manually allocated in the sparseirq case.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>