Blackfin: SMP: tweak platform_request_ipi() usage

This function takes an irq_handler_t function, but the prototype in
the header doesn't match the function definition.  This is due to the
smp headers needing to avoid circular dependencies.  So change the
function to take a simple pointer.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>