arm64: spin-table: add more information in Kconfig help

arm64: spin-table: add more information in Kconfig help

This feature seems to be sometimes misunderstood.  The intention is:

[1] Bring the slaves into the U-Boot proper image, not SPL (unless
    you have a special reason to do otherwise).

[2] The operation must be done in a board (SoC) specific manner
    since how to wake the slaves from the Boot ROM is SoC specific.

[3] The slaves must enter U-Boot proper after U-Boot relocates
    itself because the "cpu-release-addr" property points to the
    relocated memory area.

[2] is already explained in the help.  We can make [1] even clearer
by mentioning "U-Boot proper" instead of "U-Boot".  [3] is missing,
so I am adding it to the list.  Instead, "before the master CPU
jumps to the kernel" is a matter of course, so removed.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>