serial: Add note about migration to driver SCCNXP

This patch adds note about migration to driver SCCNXP in the code
of driver SC26XX and in MIPS SNI board initialization with example.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>