configs: stratix10: Enable CONFIG_SPI_FLASH_USE_4K_SECTORS

Fix SPI flash environment erase size error.

CONFIG_ENV_SECT_SIZE is set to 4KB. Enable CONFIG_SPI_FLASH_USE_4K_SECTORS
to allow erase one environment sector.

Fix error below:

SOCFPGA_STRATIX10 # saveenv
Saving Environment to SPI Flash...
SF: Detected mt25qu02g with page size 256 Bytes, erase size 64 KiB, total 256 MiB
Erasing SPI flash...SF: Erase offset/length not multiple of erase size
Failed (-22)

Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>