mmc: sdhci-msm: add support for fourth drive strength

This change adds support for new drive strength.
Before this change sdhci-msm supported only 3 types of drive
strength, starting from eMMC version 5.0 there is additional
drive strength.

Change-Id: Ib0bf945560107ec732e7ffeb7653d271d8d91c2f
Signed-off-by: Talel Shenhar <tatias@codeaurora.org>