[RFC] Add alternate timer mechanism to LiteX BIOS on POWER systems
All POWER systems have an architecturally defined decrementer and timestamp counter. On such systems, the LiteX timer peripheral is redundant, but the LiteX BIOS assumes it is present on all SoCs.
Add a minimal interface from the LiteX BIOS code to the POWER internal decrementer for SoCs without the LiteX timer peripheral.