x86: fsp: Replace e-mmc with emmc in devicetree bindings

The term eMMC is used inconsistently within the FSP devicetree
bindings (e-mmc and emmc), especially for "emmc-host-max-speed"
documentation and code disagree.

Change all eMMC instances within the FSP bindings to consistently
use "emmc". The term "emmc" is already used a lot within U-Boot,
while "e-mmc" is only used in the FSP bindings.

Signed-off-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: correct one typo in the commit message]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>