memory: tegra20: Add debug statistics

Add debug statistics collection support. The statistics is available
via debugfs in '/sys/kernel/debug/mc/stats', it shows percent of memory
controller utilization for each memory client. This information is
intended to help with debugging of memory performance issues, it already
was proven to be useful by helping to improve memory bandwidth management
of the display driver.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210319130933.23261-1-digetx@gmail.com