mmc: sdhci-msm: Add bus speed mode selection

Add bus speed mode selection using kernel command line parameter.

The module parameter "bus_mode" should be enabled during
boot up by adding this to the kernel command line -
"sdhci_msm.bus_mode=x" where x should be between 1 and 4.

1 -> HS400
2 -> HS200
3 -> DDR52
4 -> HS

Default mode is HS400ES.

Change-Id: Icfc4bff5d2fc84b39e2ef80a6a220ea19ab41952
Signed-off-by: Sarthak Garg <sartgarg@codeaurora.org>