discover: Use platform code to read sysinfo type and identifier

This change uses the platform-specific code to read the system type and
identifier, rather than the pb-sysinfo utility. We introduce a new
callback for struct platform:

  int (*get_sysinfo)(struct platform *, struct system_info *);

- which populates struct system_info with appropriate information.

This means that the system-specific code is kept in one place; rather
than having powerpc-specific device-tree-reading code in the pb-sysinfo
shell script.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>