Tegra186: mce: Avoid implementation-defined bitfield types

GCC version 4.8 (and presumably earlier) warn when non-standard types are
used for bitfield definitions when -pedantic is enabled. This prevents TF
from being built with such toolchains, since -Werror -pedantic options are
used.

gcc-4.9 removed this warning; -pedantic is intended to cause gcc to emit a
warning in all cases required by the standard, but the standard does not
require a warning in this case.

See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57773



Signed-off-by: Stephen Warren <swarren@nvidia.com>