ARM: head.S: axxia: Allow more granularity in the starting offset

commit  ab2de53f52ffccf44ab5c9900d19bbd65c8e9457 from
https://github.com/axxia/linux-yocto.git axxia-linux/v5.10/standard/axxia-dev/base

For Axxia AXM55xx, the U-Boot is kept in the bottom 8M of memory,
so the math in arch/arm/boot/compressed/head.S would "align"
8M to 0M and wipe out U-Boot.

Without this changes, Linux has to be rather severely aligned.

Signed-off-by: John Jacques <john.jacques@intel.com>
Signed-off-by: Charlie Paul <charlie.paul@windriver.com>
Signed-off-by: Daniel Dragomir <daniel.dragomir@windriver.com>
Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>