atmel-samd: UART: allocate rx buffer in long-lived region
atmel-samd: UART: allocate rx buffer in long-lived region

This is not strictly needed in order for #1056 to be resolved,
because the "make long-lived" machinery is unaware of this pointer.

However, as UARTs are assumed to be long-lived, this change is
beneficial because it moves the long-lived buffer into the upper
memory area with other long-lived objects, instead of remaining in
the low heap.

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