soc: nxp: imxrt: Initialize FlexRAM before first stack use

If the stack is in ITCM, DTCM, or FlexRAM OCRAM, flexram_dt_partition()
may change its contents. The comment on flexram_dt_partition()
acknowledges that, stating that it's inlined because it "cannot use
[the] stack". But we currently call SystemInit(), which is not inlined
and does use the stack, prior to flexram_dt_partition()! Fix that issue
by reordering the calls.

It seems to me that flexram_dt_partition() would be safer as a
soc_early_reset_hook implemented in assembly, but this fix does work for
the moment. Tested on an i.MX RT1061, with the stack in FlexRAM OCRAM
and entering Zephyr with all FlexRAM allocated to ITCM and DTCM.

Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>