mmc: omap_hsmmc: allow mmc clock to be gated

mmc core has defined a new parameter *clk_disable* to gate the clock.
Disable the clock here if *clk_disable* is set.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>