ENGR00235817 mx6: use SNVS LPGPR register to store boot mode value.

ENGR00235817 mx6: use SNVS LPGPR register to store boot mode value.

After using POR reset, the content in SRC will be reset.
See RM: 63.5.1.2.3 IPP_RESET_B(POR)

Because POR reset will reset most of register in IC, so use
SNVS_LP General Purpose Register (LPGPR) to store the boot mode value.

Below copy from SNVS_BlockGuide.pdf:
The SNVS_LP General Purpose Register provides a 32 bit read write
register, which can be used by any application for retaining 32 bit
data during a power-down mode

This Patch will use [7,8] bits of this register.

Signed-off-by: Zhang Jiejing <jiejing.zhang@freescale.com>