hikey+debian: arm-tf: Remove -O0 -gdwarf-2 flags

On upstream arm-tf repo, building with -O0 -gdwarf-2 flags causes the
size of bl2.bin to exceed HIKEY_BL1_MMC_DATA_SIZE in hikey_def.h and
trigger an assert in emmc.c.

Signed-off-by: Victor Chong <victor.chong@linaro.org>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>