ARM: shmobile: Minor update for the Lager DT reference code

Update the Lager DT reference code to get rid of the redundant
irqchip header and also make lager_add_standard_devices() static.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>