MAINTAINERS: Bestow upon myself the care for drivers/hwtracing

Although all components of drivers/hwtracing are Maintained, the
directory itself isn't, which leads to confusion when people want
to patch the Kconfig file, for example.

This adds an entry to the MAINTAINERS file with myself as a
maintainer.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>