i2c: pxa: Add definition of fast and high speed modes via the regs layout

So far, the bit masks for the fast and high speed mode were statically
defined. Some IP blocks might use different bits for these modes.

This commit introduces new fields in order to enable the definition of
different bit masks for these features. If these fields are undefined,
ICR_FM and ICR_HS are selected to preserve backward compatibility with
other IPs.

Signed-off-by: Romain Perier <romain.perier@free-electrons.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>