mfd: rc5t583: Fix array subscript is above array bounds

I got below build warning while compiling this driver.
It's obviously RC5T583_MAX_INTERRUPT_MASK_REGS is 9 but irq_en_add
array only has 8 elements.

  CC      drivers/mfd/rc5t583-irq.o
drivers/mfd/rc5t583-irq.c: In function 'rc5t583_irq_sync_unlock':
drivers/mfd/rc5t583-irq.c:227: warning: array subscript is above array bounds
drivers/mfd/rc5t583-irq.c: In function 'rc5t583_irq_init':
drivers/mfd/rc5t583-irq.c:349: warning: array subscript is above array bounds

Since the number of interrupt enable registers is 8, this patch adds
define for RC5T583_MAX_INTERRUPT_EN_REGS to fix this bug.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>