The referred config BIG_ENDIAN does not exist. The config for the
endianness of the CPU architecture is called CPU_BIG_ENDIAN.

Correct the config name to the existing config for the endianness.
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Nicolas Pitre <npitre@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220103094504.3602-1-lukas.bulwahn@gmail.com