ARM: make the physical-relative calculation more obvious

The physical-relative calculation between the XIP text and data sections
introduced by the previous patch was far from obvious. Let's simplify it
by turning it into a macro which takes the two (virtual) addresses.

This allows us to arrange the calculation in a more obvious manner - we
can make it two sub-expressions which calculate the physical address for
each symbol, and then takes the difference of those physical addresses.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>