SPI flash not working with the ECP5 when using the Lattice Diamond toolchain

[dwalton65]

Firstly #2064 needs not be addressed, as this is relevant when using trellis or diamond.

I am using a ULX3S for testing.
If the project is built using the Diamond toolchain, all reads from SPI flash return 0xff. Even reads from the ID register.
The problem appears to be the way the USRMCLK USERMCLKTS is used. The Lattice document "FPGA-TN-02039-2-2-ECP5-and-ECP5-5G-sysCONFIG.pdf" says "USRMCLKTS must be connected to a signal (it cannot be connected to a constant value)".
LiteSPIClkGen sets USRMCLKTS to 0.
If I change this line to i_USRMCLKTS = ResetSignal("sys"), the spi flash works as expected.

Setting USRMCLKTS to 0 works fine when using the trellis toolchain.