xen/arm: alternative: Clean-up __apply_alternatives

xen/arm: alternative: Clean-up __apply_alternatives

This patch contains only renaming and comment update. There are no
functional changes:
    - Don't mix _start and _stext, they both point to the same address
    but the former makes more sense (we are mapping the Xen binary, not
    only the text section).
    - s/text_mfn/xen_mfn/ and s/text_order/xen_order/ to make clear that
    we map the Xen binary.
    - Mention about inittext as alternative may patch this section.
    - Use 1U instead of 1 in shift

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>