soc: arm: nxp_imx: rt5xx: cleanup core if booting from bootloader

Cleanup core if booting from bootloader using RT5xx. This is required
because the call to SystemInit will push data to the stack, and the
bootloader may have configured stack limits or MPU settings. Either
would cause the core to fault if these settings are not first
cleaned up.

Perform this cleanup if the boot header is not present, as in this case
the application was likely kicked off via a bootloader.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>