drivers: dma: dma_nxp_edma: query channel number from HAL config

Currently, the number of channels supported by the controlled
is computed based on the size of the channel array. This
works well only if there's no gaps (i.e: "dma-channels" property
is used or "valid-channels" property is used with contiguous
channels) but will break if there are any gaps. For instance,
if the user wants to use channels 16 and 17 and specifies them
through the "valid-channels" property, they won't be allowed
to do so because dma_request_channels() will stop at channel 1.
As such, to fix this, simply use the number of channels from
the HAL configuration which is the maximum number of channels.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>