mmc: core: Enable MMC_CAP2_CACHE_CTRL as default

There are no reason to why the use of a non-volatile internal eMMC
cache should be controlled by a host cap. Instead let's just enable it
if the eMMC card supports it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <chris@printf.net>