nios2: remove EP1C20, EP1S10, EP1S40 boards

The example configuration files of nios2-generic board can generated
binary to run on the EP1C20, EP1S10, and EP1S40 boards. So the three
boards can be removed.

With nios2-generic approach, the fpga parameter header file can
be generated from hardware designs using tools. Porting u-boot for
nios2 boards is simplified. Vendors can supply their fpga parameter
file or patches to add a new nios2-generic board instance. There is
no need to include other boards support for nios2 in the u-boot
mainline.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Scott McNutt <smcnutt@psyent.com>