firmware: imx: scu-pd: Rename mu PD range to mu_a

The Messaging Unit module enables two processors within the SoC to
communicate and coordinate by passing messages through the MU interface.

MUs have 2 “sides” with independent programming interfaces. Rename
mu PD range to mu_a because it's actually side A of MUs.

Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>