imx: bootaux: change names of MACROs used to boot MCU on iMX devices

The current bootaux supports i.MX8M and i.MX93, but the name "_M4_"
implies that the SoCs have Cortex-M4. Actually i.MX8MM/Q use Cortex-M4,
i.MX8MN/P use Cortex-M7, i.MX93 use Cortex-M33, so use "_MCU_" in place
of "_M4_" to simplify the naming.

Signed-off-by: faqiang.zhu <faqiang.zhu@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>