arm64: versal-net: Enable defconfig for Micron octal flashes

Micron mt35 series octal flashes are under config option
CONFIG_SPI_FLASH_MT35XU. Enable it in default defconfig for octal
flashes to be detected.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@amd.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/dfaf73a72c7b408f1b8530f411c405d3dcf9f854.1669717110.git.michal.simek@amd.com