fix BLK_DEV_HD_ONLY on ARM dependencies

This patch limits BLK_DEV_HD_ONLY to the ARM platforms offering 
IRQ_HARDDISK, fixing the following compile error on others:

<--  snip  -->

...
  CC      drivers/ide/legacy/hd.o
...
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/ide/legacy/hd.c: In function 'hd_times_out':
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/ide/legacy/hd.c:542: error: 'IRQ_HARDDISK' undeclared (first use in this function)
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/ide/legacy/hd.c:542: error: (Each undeclared identifier is reported only once
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/ide/legacy/hd.c:542: error: for each function it appears in.)
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/ide/legacy/hd.c: In function 'do_hd_request':
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/ide/legacy/hd.c:661: error: 'IRQ_HARDDISK' undeclared (first use in this function)
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/ide/legacy/hd.c: In function 'hd_init':
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/ide/legacy/hd.c:765: error: 'IRQ_HARDDISK' undeclared (first use in this function)
make[3]: *** [drivers/ide/legacy/hd.o] Error 1

<--  snip  -->

Reported-by: Adrian Bunk <bunk@kernel.org>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>