nios2: rename board nios2-generic to 3c120_devboard

Rename board nios2-generic to 3c120_devboard. Since nios2 is
converted to driver model and device tree control of u-boot,
the nios2-generic board directory is removed. We can rename
the board back to a real board name. Now the boards maintained
in u-boot mainline are the same as Linux kernel, namely 3c120
and 10m50.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Marek Vasut <marex@denx.de>

configs/nios2-generic_defconfig → configs/3c120_defconfig

 CONFIG_NIOS2=y
-CONFIG_SYS_CONFIG_NAME="nios2-generic"
+CONFIG_SYS_CONFIG_NAME="3c120_devboard"
 CONFIG_DM_SERIAL=y
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="3c120_devboard"