Escape asterisk symbols to fix the following warning:

"WARNING: Inline emphasis start-string without end-string"

Fixes: c6807970 ("soc: aspeed: Add UART routing support")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20220124014351.9121-1-chiawei_wang@aspeedtech.com
Link: https://lore.kernel.org/r/20220201070027.196314-1-joel@jms.id.au'
Signed-off-by: Arnd Bergmann <arnd@arndb.de>