fsi: Aspeed: Add mutex to protect HW access

There is nothing to prevent multiple commands being executed
simultaneously. Add a mutex to prevent this.

Fixes: 606397d6 ("fsi: Add ast2600 master driver")
Reviewed-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Milton Miller <miltonm@us.ibm.com>
Signed-off-by: Eddie James <eajames@linux.ibm.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20201120004929.185239-1-joel@jms.id.au


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>