mmc: sdhci-of-aspeed: Allow max-frequency limitation of SDCLK

Add a get_max_clock() handler to sdhci-of-aspeed to report f_max as the
maximum clock rate if it is set. This enables artificial limitation of
the bus speed via max-frequency in the devicetree for e.g. the AST2600
evaluation board where I was seeing errors at 200MHz.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>