x86: tsc: Update comments and expand definitions in freq_desc_tables[]

Some processor abbreviations in the comments of freq_desc_tables[]
are obscure. This updates part of these to mention processors
that are known to us. Also expand frequency definitions.

This keeps in sync with Linux kernel commit:
  9e0cae9: x86/tsc_msr: Update comments, expand definitions

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>