eeprom: at25: convert to spi-mem API

Replace the RAW SPI accesses with spi-mem API. The latter will fall back to
RAW SPI accesses if spi-mem callbacks are not implemented by a controller
driver.

Notable advantages:
- read function now allocates a bounce buffer for SPI DMA compatibility,
  similar to write function;
- the driver can now be used in conjunction with SPI controller drivers
  providing spi-mem API only, e.g. spi-nxp-fspi.
- during the initial probe the driver polls busy/ready status bit for 25ms
  instead of giving up instantly and hoping that the FW didn't write the
  EEPROM

Notes:
- mutex_lock() has been dropped from fm25_aux_read() because the latter is
  only being called in probe phase and therefore cannot race with
  at25_ee_read() or at25_ee_write()

Quick 4KB block size test with CY15B102Q 256KB F-RAM over spi_omap2_mcspi
driver (no spi-mem ops provided, fallback to raw SPI inside spi-mem):

OP	| throughput, KB/s	| change
--------+-----------------------+-------
write	| 1717.847 -> 1656.684	| -3.6%
read	| 1115.868 -> 1059.367	| -5.1%

The lower throughtput probably comes from the 3 messages per SPI transfer
inside spi-mem instead of hand-crafted 2 messages per transfer in the
former at25 code. However, if the raw SPI access is not preserved, then
the driver doesn't grow from the lines-of-code perspective and subjectively
could be considered even a bit simpler.

Higher performance impact on the read operation could be explained by the
newly introduced bounce buffer in read operation. I didn't find any
explanation or guarantee, why would a bounce buffer be not needed on the
read side, so I assume it's a pure luck that nobody read EEPROM into
some variable on stack on an architecture where kernel stack would be
not DMA-able.

Cc: Michael Walle <mwalle@kernel.org>
Cc: Hui Wang <hui.wang@canonical.com>
Link: https://lore.kernel.org/all/28ab8b72afee1af59b628f7389f0d7f5@kernel.org/
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Link: https://lore.kernel.org/r/20250702222823.864803-1-alexander.sverdlin@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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