ARM: shmobile: mackerel: Add FLCTL IRQ resource

Since commit 3c7ea4e (mtd: sh_flctl: Add support for error IRQ)
the sh_flctl driver requires the error IRQ line to signal failed
transactions between the flash controller and the NAND chip.
This information is mandatory - else the driver refuses to start
up. We provide it here for the board mackerel.

Signed-off-by: Bastian Hecht <hechtb@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>