lpc: Add usart shim driver for lpcxpresso54114

Add usart-yaml in dts.

Build fsl_lpc_usart and fsl_lpc_flexcomm in
ext/hal/nxp/mcux/drivers/Makefile.

Only polling mode is implemented in usart now. Interrupt can be added in
future.

Signed-off-by: Shiksha Patel <shiksha.patel@nxp.com>
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>