openrisc: Implement fixmap to fix earlycon

[ Upstream commit 1037d186edfc551fa7ba2d4336e74e7575a07a65 ]

With commit 53c98e35 ("openrisc: mm: remove unneeded early ioremap
code") it was commented that early ioremap was not used in OpenRISC.  I
acked this but was wrong, earlycon was using it.  Earlycon setup now
fails with the below trace:

    Kernel command line: earlycon
    ------------[ cut here ]------------
    WARNING: CPU: 0 PID: 0 at mm/ioremap.c:23
    generic_ioremap_prot+0x118/0x130
    Modules linked in:
    CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted
    6.11.0-rc5-00001-gce02fd891c38-dirty #141
    Call trace:
    [<(ptrval)>] dump_stack_lvl+0x7c/0x9c
    [<(ptrval)>] dump_stack+0x1c/0x2c
    [<(ptrval)>] __warn+0xb4/0x108
    [<(ptrval)>] ? generic_ioremap_prot+0x118/0x130
    [<(ptrval)>] warn_slowpath_fmt+0x60/0x98
    [<(ptrval)>] generic_ioremap_prot+0x118/0x130
    [<(ptrval)>] ioremap_prot+0x20/0x30
    [<(ptrval)>] of_setup_earlycon+0xd4/0x2e0
    [<(ptrval)>] early_init_dt_scan_chosen_stdout+0x18c/0x1c8
    [<(ptrval)>] param_setup_earlycon+0x3c/0x60
    [<(ptrval)>] do_early_param+0xb0/0x118
    [<(ptrval)>] parse_args+0x184/0x4b8
    [<(ptrval)>] ? start_kernel+0x0/0x78c
    [<(ptrval)>] parse_early_options+0x40/0x50
    [<(ptrval)>] ? do_early_param+0x0/0x118
    [<(ptrval)>] parse_early_param+0x48/0x68
    [<(ptrval)>] ? start_kernel+0x318/0x78c
    [<(ptrval)>] ? start_kernel+0x0/0x78c
    ---[ end trace 0000000000000000 ]---

To fix this we could either implement early_ioremap again or implement
fixmap.  In this patch we choose the later option of implementing basic
fixmap support.

While fixing this we also remove the old FIX_IOREMAP slots that were
used by early ioremap code.  That code was also removed by commit
53c98e35 ("openrisc: mm: remove unneeded early ioremap code") but
these definitions were not cleaned up.

Fixes: 53c98e35 ("openrisc: mm: remove unneeded early ioremap code")
Signed-off-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>