mmc: sdhci-bcm-kona: kill the "external_clk" member in driver private struct

Actually we can use the "clk" in the struct sdhci_pltfm_host. Also
change the "external clock" to "core clock" and kill two redundant
private functions in this driver as suggested by Ray Jui.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Reviewed-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>