regmap-i2c: add SMBus byte/word reg16 bus for adapters lacking I2C_FUNC_I2C

regmap-i2c: add SMBus byte/word reg16 bus for adapters lacking I2C_FUNC_I2C

AMD PIIX4 SMBus adapters, present on AMD SP5/EPYC-based platforms
(including Cisco 8000 series routers), support SMBUS_BYTE_DATA and
SMBUS_WORD_DATA but lack I2C_FUNC_I2C and I2C_FUNC_SMBUS_I2C_BLOCK.

When at24 (or any driver) requests a regmap with reg_bits=16 and
val_bits=8 on such an adapter, regmap_get_i2c_bus() finds no matching
bus and returns -ENOTSUPP.  The existing regmap_i2c_smbus_i2c_block_reg16
bus type already implements 16-bit addressed reads using only
write_byte_data() + read_byte() primitives, but its selection is gated
on I2C_FUNC_SMBUS_I2C_BLOCK which these adapters lack.

Add a new regmap_smbus_byte_word_reg16 bus that:

  READ:  reuses regmap_i2c_smbus_i2c_read_reg16() -- sets the 16-bit
         address via write_byte_data(addr_lo, addr_hi), then reads
         bytes sequentially via read_byte() (EEPROM auto-increments).
         Requires only SMBUS_BYTE_DATA.

  WRITE: uses write_word_data(addr_hi, (data << 8) | addr_lo) to
         encode one data byte per SMBus WORD transaction.
         Requires only SMBUS_WORD_DATA.  Single-byte writes only.

The new bus is selected in regmap_get_i2c_bus() when reg_bits=16,
val_bits=8, and the adapter has SMBUS_BYTE_DATA | SMBUS_WORD_DATA but
not I2C_FUNC_I2C or SMBUS_I2C_BLOCK.  The branch is placed after the
existing I2C_BLOCK_reg16 check so adapters with full block support
continue to use the faster path.

This fixes at24 EEPROM probe failures on PIIX4:
  at24 3-0055: probe with driver at24 failed with error -524

No driver changes are required -- at24 already passes reg_bits=16 to
devm_regmap_init_i2c(), which now succeeds.

Signed-off-by: Nishanth Sampath Kumar <nissampa@cisco.com>
Link: https://patch.msgid.link/20260407233927.498932-1-nissampa@cisco.com
Signed-off-by: Mark Brown <broonie@kernel.org>