mmc: core: Remove BROKEN_CLK_GATING quirk for AR6003 cards

MMC_CLKGATE feature can be enabled for Atheros AR6003 cards as
the host supports asynchronous acitivity notification when the
clocks are off.

Change-Id: Ieff00382894a0841496776dc6a6b1db53e204d6d
Signed-off-by: Krishna Konda <kkonda@codeaurora.org>