clk: aspeed: Support second reset register

The ast2500 has an additional reset register that contains resets not
present in the ast2400. This enables support for this register, and adds
the one reset line that is controlled by it.

Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>