mmc: core: export mmc_host_may_gate_card function

MMC Host driver might want to use the mmc_host_may_gate_card() API
to know whether the clock can be gated to the card or not hence
export this function for usage outside its current scope.

Change-Id: I9ec4173063e2505eca179161cdf8cda033a3fd4b
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>