xea: defconfig: Update defconfig to support mtd partitions r/w by name

After this change it would be possible to use 'mtd' command to get access
to XEA's SPI-NOR partitions by name (e.g. SPL), not by offsets.

To enable this feature the CONFIG_SPI_FLASH_MTD needs to be defined in the
Kconfig, not in xea.h.

=> mtd list
=> mtd read spl-boot-data1 ${loadaddr} 0x0 4
=> md.l ${loadaddr} 1

Signed-off-by: Lukasz Majewski <lukma@denx.de>