xen/arm64: head: Rework and document zero_bss()

xen/arm64: head: Rework and document zero_bss()

On secondary CPUs, zero_bss() will be a NOP because BSS only need to be
zeroed once at boot. So the call in the secondary CPUs path can be
removed. It also means that x26 does not need to be set for secondary
CPU.

Note that we will need to keep x26 around for the boot CPU as BSS should
not be reset when booting via UEFI.

Lastly, document the behavior and the main registers usage within the
function.

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