clk: AST2600: Add mux for EMMC clock

The EMMC clock can be derived from either the HPLL or the MPLL. Register
a clock mux so that the rate is calculated correctly based upon the
parent.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20200709195706.12741-2-eajames@linux.ibm.com
Acked-by: Joel Stanley <joel@jms.id.au>
Fixes: d3d04f6c

 ("clk: Add support for AST2600 SoC")
Signed-off-by: Stephen Boyd <sboyd@kernel.org>