soc: aspeed: Add UART routing support

Add driver support for the UART routing control. Users can perform
runtime configuration of the RX muxes among the UART controllers and
the UART IO pins.

The sysfs interface is also exported for the convenience of routing paths
check and update.

Signed-off-by: Oskar Senft <osk@google.com>
Signed-off-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Tested-by: Lei YU <yulei.sh@bytedance.com>
Link: https://lore.kernel.org/r/20210927023053.6728-5-chiawei_wang@aspeedtech.com
Link: https://lore.kernel.org/r/20211022000616.481772-1-joel@jms.id.au

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>