Add UART and DMA drivers

Both blocking and async versions are supported. Add separate examples
for each mode.

docs/pages/imxrt.adoc

@@ -10,5 +10,7 @@ The link: link:https://github.com/embassy-rs/embassy/tree/main/embassy-imxrt[Emb
 The following peripherals have a HAL implementation at present
 
 * CRC
+* DMA
 * GPIO
 * RNG
+* UART