Change the default number of performance counters from 0 to 10 and replace all instances of Ibex with cve2.

[cairo-caplan]

… (#214)

rtl/cve2_cs_registers.sv did not need to be changed as its default value was already 10

[MikeOpenHWGroup]

LGTM @cairo-caplan. Since we are editing rtl/cve2_top.sv and rtl/cve2_core.sv already, can you replace "ibex" with "cve2" in the top-level comment?