ARM: mxs: Get boot mode from OCRAM

Reading the boot mode pins after power-up does not necessarily represent the
boot mode used by the ROM loader. For example the state of a pin may have
changed because a recovery switch which was pressed to enter USB mode is
already released after plugging in USB.

The ROM loader stores the value a fixed address in OCRAM. Use this value
instead of reading the boot map pins.

The GLOBAL_BOOT_MODE_ADDR for i.MX28 is taken from an U-Boot patch for the
MX28EVK:
http://repository.timesys.com/buildsources/u/u-boot/u-boot-2009.08/u-boot-2009.08-mx28-201012211513.patch

Leave the boot mode detection for the i.MX23 untouched. Someone has to test
whether the i.MX ROM loader does also store the boot mode in OCRAM and if the
address match.

This patch superseeds my incorrect patch:
ARM: mxs: get boot mode from OTP
http://patchwork.ozlabs.org/patch/454930/



Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Cc: Stefano Babic <sbabic@denx.de>