mem-ruby: Fix maskLowOrderBits

mem-ruby: Fix maskLowOrderBits

The function was wrong when number = 63. Also, use the more reliable
src/base/bitfield.hh's mbits when posible.

maskLowOrderBits has only been kept because SLICC does not accept
a templated function.

Change-Id: I8dd680da02ceb9e614e2f9cbf8f1ac52cead8d45
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21084
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>