ARM: shmobile: Rework multiplatform include workaround

Now when the majority of the include files have moved from
arch/arm/mach-shmobile/include/mach/ to arch/arm/mach-shmobile/
remove the header include file workaround in the Makefile...

... and add another workaround in irqs.h to cope with the fact
that <mach/irqs.h> needs to be where it is until the PFC code
has been updated to remove legacy non-DT interfaces.

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