apalis/colibri_t30: add note about colibri vs. nvidia uart mapping

The following mapping is applicable for Apalis T30:

Apalis UART1: NVIDIA UARTA
Apalis UART2: NVIDIA UARTD
Apalis UART3: NVIDIA UARTB
Apalis UART4: NVIDIA UARTC

The following mapping is applicable for Colibri T30:

Colibri UART-A: NVIDIA UARTA
Colibri UART-B: NVIDIA UARTD
Colibri UART-C: NVIDIA UARTB

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>