ARM: imx6ul: populates platform device at .init_machine

ARM: imx6ul: populates platform device at .init_machine

At imx6ul_init_machine, it calls imx6ul_pm_init which needs to find
platform device for ocram, but the default populate platform device is at
of_platform_default_populate_init, which is located at arch_initcall_sync,
and called later than arch_initcall (.init_machine is located at that).
So below warning message will be showed during boots up:

	imx6q_suspend_init: failed to find ocram device!
	imx6_pm_common_init: No DDR LPM support with suspend -19!

Due to lack of ocram device, the suspend mode "mem" which needs ocram to
store suspend routine code is invalid.

This commit populates platform device before imx6ul_pm_init like other
imx6 platforms do, and the suspend mode "mem" can work successfully.

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Fixes: 850bea2335e4 ("arm: Remove unnecessary of_platform_populate with default match table")
Cc: Anson Huang <anson.huang@nxp.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Rob Herring <robh@kernel.org>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>