Now that we have common definitions for the performance monitor register
encodings, make the KVM code use these, simplifying the sys_reg_descs
table.

The comments for PMUSERENR_EL0 and PMCCFILTR_EL0 are kept, as these
describe non-obvious details regarding the registers. However, a slight
fixup is applied to bring these into line with the usual comment style.
Signed-off-by: NMark Rutland <mark.rutland@arm.com>
Acked-by: NChristoffer Dall <christoffer.dall@linaro.org>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: kvmarm@lists.cs.columbia.edu