ata: ahci_brcm: add compile test support

Add Kconfig dependendy on COMPILE_TEST to allow compile tests with
configs that do not enable ARCH_BRCMSTB, BMIPS_GENERIC or ARCH_BCM_XXX.

Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>