i2c: designware: Add comment to custom register value constants

DW_IC_COMP_VERSION register contains the ASCII representation of the
Synopsys component version. Here 0x3131312A == "111*" means version
1.11* required for DW_IC_SDA_HOLD register availability where '*' means
any letter starting from 'a'.

DW_IC_COMP_TYPE is constant and is derived from two ASCII letters "DW"
followed by a 16-bit unsigned number.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>