irqdomain: Change the type of 'size' in __irq_domain_add() to be consistent

The 'size' is used in struct_size(domain, revmap, size) and its input
parameter type is 'size_t'(unsigned int).
Changing the size to 'unsigned int' to make the type consistent.

Signed-off-by: Bixuan Cui <cuibixuan@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210916025203.44841-1-cuibixuan@huawei.com