mmc: sdhci-of-aspeed: Add AST2600 bus clock support

The AST2600 can achieve HS200 speeds with a change to the bus clock
divisor behaviour. The divisor can also be more accurate with respect
to the requested clock rate, but keep the one-hot behaviour for
backwards compatibility with the AST2400 and AST2500.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20210114031433.2388532-4-andrew@aj.id.au


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>