aspeed/i2c: Add support for pool buffer transfers

aspeed/i2c: Add support for pool buffer transfers

The Aspeed I2C controller can operate in different transfer modes :

  - Byte Buffer mode, using a dedicated register to transfer a
    byte. This is what the model supports today.

  - Pool Buffer mode, using an internal SRAM to transfer multiple
    bytes in the same command sequence.

Each SoC has different SRAM characteristics. On the AST2400, 2048
bytes of SRAM are available at offset 0x800 of the controller AHB
window. The pool buffer can be configured from 1 to 256 bytes per bus.

On the AST2500, the SRAM is at offset 0x200 and the pool buffer is of
16 bytes per bus.

On the AST2600, the SRAM is at offset 0xC00 and the pool buffer is of
32 bytes per bus. It can be splitted in two for TX and RX but the
current model does not add support for it as it it unused by known
drivers.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Tested-by: Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20191119141211.25716-2-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>