dmaengine: pxa_dma: use platform_get_irq_optional

dmaengine: pxa_dma: use platform_get_irq_optional

The first IRQ is required, but IRQs 1 through (nb_phy_chans - 1) are
optional, because on some platforms (e.g. PXA168) there is a single IRQ
shared between all channels.

This change inhibits a flood of "IRQ index # not found" messages at
startup. Tested on a PXA168-based device.

Fixes: 682a742ed79d ("driver core: platform: Add an error message to platform_get_irq*()")
Signed-off-by: Doug Brown <doug@schmorgal.com>
Link: https://lore.kernel.org/r/20220906000709.52705-1-doug@schmorgal.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>