irqchip/spear-shirq: Add support for IRQ 0..6

IRQ 0..7 are not supported by the driver for SPEAr320 SOC family.

IRQ 0 is not reserved in SPEAr320 SOC (assigned to GPIOINT).
Furthermore, in SPEAr320s SOC variant, IRQ 0..6 are assigned
as follow:
  IRQ 6 - NGPIO_INTR: Combined status of edge programmable
                      interrupts from GPIO ports
  IRQ 5 - TX_OR_INTR: I2S interrupt on Transmit FIFO overrun
  IRQ 4 - TX_EMP_INTR: I2S interrupt on Transmit FIFO empty
  IRQ 3 - RX_OR_INTR: I2S interrupt on Receive FIFO overrun
  IRQ 2 - RX_DA_INTR: I2S interrupt on data available in Receive FIFO
  IRQ 1 - Reserved
  IRQ 0 - GPIO_INTR: Legacy interrupt from GPIO ports

Add support for these IRQs in SPEAr320 SOC family.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211202095255.165797-6-herve.codina@bootlin.com