UPSTREAM: ARM: 8318/1: treat CPU feature register fields as signed quantities

The various CPU feature registers consist of 4-bit blocks that
represent signed quantities, whose positive values represent
incremental features, and whose negative values are reserved.

To improve forward compatibility, update the feature detection
code to take possible future higher values into account, but
ignore negative values.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
(cherry picked from commit b8c9592b4a6c93211c8163888a97880d608503b5)
Change-Id: Ib43b6b4a24015704ed57bbe536af8ad07530e246
Signed-off-by: Eric Biggers <ebiggers@google.com>