i2c: aspeed: Retain delay/setup/hold values when configuring bus frequency

In addition to the base, low and high clock configuration, the AC timing
register #1 on the AST2400 houses fields controlling:

1. tBUF: Minimum delay between Stop and Start conditions
2. tHDSTA: Hold time for the Start condition
3. tACST: Setup time for Start and Stop conditions, and hold time for the
   Repeated Start condition

These values are defined in hardware on the AST2500 and therefore don't
need to be set.

aspeed_i2c_init_clk() was performing a direct write of the generated
clock values rather than a read/mask/modify/update sequence to retain
tBUF, tHDSTA and tACST, and therefore cleared the tBUF, tHDSTA and tACST
fields on the AST2400. This resulted in a delay/setup/hold time of 1
base clock, which in some configurations is not enough for some devices
(e.g. the MAX31785 fan controller, with an APB of 48MHz and a desired
bus speed of 100kHz).

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>