ACPI: processor: remove comment regarding string _UID support

ACPI 6.3 Errata A no longer allows _UID to return a string except for
Itanium (for historical reasons) as stated in section 5.2.12:

"From ACPI Specification 6.3 onward, all processor objects for all
architectures except Itanium must now use Device() objects with an
_HID of ACPI0007, and use only integer _UID values."

Therefore, the "we don't handle string _UIDs yet" comment, which
implies a missing feature, is redundant, so drop it.

Signed-off-by: Alex Hung <alex.hung@canonical.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>