m68k: Dispatch nvram_ops calls to Atari or Mac functions

A multi-platform kernel binary has to decide at run-time how to dispatch
the arch_nvram_ops calls. Add a platform-independent arch_nvram_ops
struct for this, to replace the atari-specific one.

Enable CONFIG_HAVE_ARCH_NVRAM_OPS for Macs.

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

m68k/atari: Implement arch_nvram_ops methods and enable CONFIG_HAVE_ARCH_NVRAM_OPS

Atari RTC NVRAM uses a checksum so implement the remaining arch_nvram_ops
methods for the set_checksum and initialize ioctls. Enable
CONFIG_HAVE_ARCH_NVRAM_OPS.

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

m68k/atari: Implement arch_nvram_ops struct

By implementing an arch_nvram_ops struct, a platform can re-use the
drivers/char/nvram.c module without needing any arch-specific code
in that module. Atari does so here.

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

nvram: Replace nvram_* function exports with static functions

Replace nvram_* functions with static functions in nvram.h. These will
become wrappers for struct nvram_ops method calls.

This patch effectively disables existing NVRAM functionality so as to
allow the rest of the series to be bisected without build failures.
That functionality is gradually re-implemented in subsequent patches.

Replace the sole validate-checksum-and-read-byte sequence with a call to
nvram_read() which will gain the same semantics in subsequent patches.

Remove unused exports.

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

m68k/atari: Move Atari-specific code out of drivers/char/nvram.c

Move the m68k-specific code out of the driver to make the driver generic.

I've used 'SPDX-License-Identifier: GPL-2.0+' for the new file because the
old file is covered by MODULE_LICENSE("GPL").

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>