xen/arm: Move fixmap definitions in a separate header

xen/arm: Move fixmap definitions in a separate header

To use properly the fixmap definitions, their user would need
also new to include <xen/acpi.h>. This is not very great when
the user itself is not meant to directly use ACPI definitions.

Including <xen/acpi.h> in <asm/config.h> is not option because
the latter header is included by everyone. So move out the fixmap
entries definition in a new header.

Take the opportunity to also move {set, clear}_fixmap() prototypes
in the new header.

Note that most of the definitions in <xen/acpi.h> now need to be
surrounded with #ifndef __ASSEMBLY__ because <asm/fixmap.h> will
be used in assembly (see EARLY_UART_VIRTUAL_ADDRESS).

The split will become more helpful in a follow-up patch where new
fixmap entries will be defined.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>