tests: uart_async: Define UART loopback peripherals for LPUART

define uart peripheral that can be connected as a loopback using
external jumper for all boards this test can be run against.
Additionally, all RT boards require DMA memory to be noncacheable. Move
SRAM to DTCM for all RT10xx series boards, and to noncacheable OCRAM for
RT11xx series.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>