Register subclasses for each ARM CPU implementation.

Let arm_cpu_list() enumerate CPU subclasses in alphabetical order,
except for special value "any".

Replace cpu_arm_find_by_name()'s string -> CPUID lookup by storing the
CPUID (aka MIDR, Main ID Register) value in the class.
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>