Add designs/uart-mux-support.md

Document the uart mux support design.

Change-Id: I40deb1f5b5f2f5d4386af769730ebfdde525820f
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
Signed-of

Add designs/uart-mux-support.md

Document the uart mux support design.

Change-Id: I40deb1f5b5f2f5d4386af769730ebfdde525820f
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>

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