ArmPlatformPkg/ArmJuno: Correct AXI->PCIe translation comments

ArmPlatformPkg/ArmJuno: Correct AXI->PCIe translation comments

The AXI<->PCIe translation comments are out of date with
respect to the code. In the first case the AXI master port
is incorrectly called a slave. In the second case the the
translation direction indicated for the slave port is the
wrong direction.

Correct both of these comments to reflect what the code is
doing.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>