Documentation: arm64/acpi : clarify arm64 support of IBFT

In commit 94bccc34

 ("iscsi_ibft: make ISCSI_IBFT dependson ACPI instead
of ISCSI_IBFT_FIND") Kconfig was disentangled to make ISCSI_IBFT selection
not depend on x86.

Update arm64 acpi documentation, changing IBFT support status from
"Not Supported" to "Optional".
Opportunistically re-flow paragraph for changed lines.

Link: https://lore.kernel.org/lkml/1563475054-10680-1-git-send-email-thomas.tai@oracle.com/

Signed-off-by: Tom Saeger <tom.saeger@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Link: https://lore.kernel.org/r/9efc652df2b8d6b53d9acb170eb7c9ca3938dfef.1615920441.git.tom.saeger@oracle.com
Signed-off-by: Will Deacon <will@kernel.org>