up5k_rgb: Fix icetime failure

I get this failure when building the up5k_rgb example:

    ERROR: Max frequency for clock 'clk': 38.87 MHz (FAIL at 48.00 MHz)

(Tested with yosys 0.23 and nextpnr 0.4)

Annotate the clk line frequency to fix this.

examples/up5k_rgb/rgb.pcf

@@ -1,3 +1,4 @@
+set_frequency clk 32
 set_io RGB0 39
 set_io RGB1 40
 set_io RGB2 41