armv7: Add Position Independent Execution support

armv7: Add Position Independent Execution support

A U-Boot image could be loaded and executed at a different
location than it was linked at.

For example, Aspeed takes a stable release version of U-Boot image
as the golden one for recovery purposes. When the primary storage
such as flash is corrupted, the golden image would be loaded to any
SRAM/DRAM address on demands through ethernet/UART/etc and run for
rescue.

To deal with this condition, the PIE is needed as there is only one
signed, golden image, which could be however executed at different
places.

This patch adds the PIE support for ARMv7 platform.

Signed-off-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>