tests: dma loop_transfer: Ex of LPC DMA Kconfig

Show an example of the LPC DMA Kconfig to reduce RAM based on the number
of DMA channels expected to be used. In this test, only one DMA channel
is needed, so we can show an example of how to reduce RAM usage of the
driver by configuring this value appropriately.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>