docs: genericirq.rst: don't document chip.c functions twice

Currently, kernel/irq/chip.c is included twice, one for
export functions, and then for internal ones. However, as
the :export:  and :internal: tags are missing, they ended
being included twice.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>