imx-atf: Remove -O2 from CFLAGS for 8MQ
The i.MX 8M Quad has only 64 kB of OCRAM, and the following build break
is observed:
```
| aarch64-poky-linux-ld.bfd: /opt/work/upstream/fsl-xwayland/tmp/work/cortexa53-crypto-mx8m-fsl-linux/imx-atf/2.4+gitAUTOINC+
ec35fef92b-r0/git/build-optee/imx8mq/release/bl31/bl31.elf section `coherent_ram' will not fit in region `RAM'
| aarch64-poky-linux-ld.bfd: BL31 image has exceeded its limit.
| aarch64-poky-linux-ld.bfd: region `RAM' overflowed by 4096 bytes
```
Fix the break by disabling -O2 optimization.
Signed-off-by: Tom Hochstein <tom.hochstein@nxp.com>
