ARM: u300: Update MMC configs for u300 defconfig

Enable MMC_UNSAFE_RESUME to be accomplish a proper suspend/resume cycle
for SD/SDIO/(e)MMC.

ARMMMCI host driver supports clock gating through runtime PM, thus
MMC_CLKGATE is not needed. Moreover ARMMMCI can do scatter-gather which
means we can explicity disable MMC_BLOCK_BOUNCE, since it's default
enabled, to skip unnecessary bounce buffer copying.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>