clk: aspeed: Add 24MHz fixed clock

Add a 24MHz fixed clock.
This clock will be used for certain devices, e.g. pwm.

Signed-off-by: Lei YU <mine260309@gmail.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>