ti_armv7_keystone2: Update addr_mon variable

As boot monitor contains a mkimage header, it can be loaded at any location.
So, have a common addr_mon address across all keystone2 SoCs. And also
making sure that boot monitor is installed early during default boot to
avoid any overlapping with other images.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>