Reduce the size of icache to help yosys ECP5 builds (#303)

Reduce the size of icache to help yosys ECP5 builds (#303)

The icache RAM is currently LUT ram not block ram. This massively
bloats the icache size. We think this is due to yosys not inferencing
the RAM correctly but that's yet to be confirmed.

Work around this for now by reducing the default size of the icache
RAM for the ECP5 builds.

On the ECP5 85K builts, this gets us from 95% down to 76% and helps
our CI to pass.

Signed-off-by: Michael Neuling <mikey@neuling.org>