mmc: dw_mmc: move supports-highspeed of quirks to caps

'supports-highspeed' is not one of the quirks but is a capability.
So, it's removed from quirks.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Tested-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>