xen/arm64: head.S: Introduce macro to load the physical address of a symbol

xen/arm64: head.S: Introduce macro to load the physical address of a symbol

A lot of places in the ARM64 assembly code requiring to load the
physical address of a symbol. Rather than open-coding the translation,
introduce a new macro that will load the physical address of a symbol.

Lastly, use this new macro to replace all the current opencoded version.

Note that most of comments associated to the code changed have been
removed because the code is now self-explanatory.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>