Gowin. Add an energy saving primitive

We add a BANDGAP primitive used to turn off power to OSC, PLL and other
things on some GOWIN chips.

We also mark this primitive and GSR as keep.

Signed-off-by: YRabbit <rabbit@yrabbit.cyou>

techlibs/gowin/cells_xtra.v

@@ -1687,10 +1687,6 @@ endmodule
 module ADC (...);
 endmodule
 
-module BANDGAP (...);
-input BGEN;
-endmodule
-
 module CLKDIV2 (...);
 parameter GSREN = "false"; 
 input HCLKIN, RESETN;