ARM: OMAP: fix MMC2 loopback clock handling

Currently MMC2 setup code can only enable loopback clock and
relies on reset value for boards that need to have it disabled.
This causes a problem with certain bootloaders that always enable
that clock, resulting with unwanted bootloader dependencies.

Fix this by making it disable the clock if board data says so.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Tony Lindgren <tony@atomide.com>