soc: fsl: fix section mismatch build warnings

We get the following warning with the driver is compiled in:

WARNING: modpost: Found 1 section mismatch(es).
To see full details build your kernel with:
'make CONFIG_DEBUG_SECTION_MISMATCH=y'

With CONFIG_DEBUG_SECTION_MISMATCH enabled, the details are reported:

WARNING: vmlinux.o(.text+0x55d014): Section mismatch in reference from the
function fsl_guts_probe() to the function
.init.text:of_flat_dt_get_machine_name()
The function fsl_guts_probe() references
the function __init of_flat_dt_get_machine_name().
This is often because fsl_guts_probe lacks a __init
annotation or the annotation of of_flat_dt_get_machine_name is wrong.

This patch fixes the issue by using the normal DT/OF API rather than
the of_flat_* one.

Cc: Scott Wood <oss@buserror.net>
Cc: Yangbo Lu <yangbo.lu@nxp.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>