orion_nand_read_buf uses an inline assembly with the "ldrd"
instruction, which is only available from ARMv5 upwards. This
used to be fine, since all users have an ARMv5 or ARMv7 CPU,
but now we can also build a multiplatform kernel with ARMv4
support enabled in addition to the "kirkwood" (mvebu) platform.

This provides an alternative to call the readsl() function that
is supposed to have the same effect and is also optimized for
performance.

I first posted a version of this patch back in 2014, and there
was some discussion about it then. This fixes the bugs identified
back then and should be a reasonable alternative for the rare
corner case.

Link: https://patchwork.kernel.org/patch/4144791/
Cc: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>