siemens-am33x-common: Hardcoded value instead of non-included define

siemens-am33x-common: Hardcoded value instead of non-included define

The config file for the siemens-am33x-common was using OMAP_I2C_STANDARD, which
is defined in a header that is not included in the config header. In most cases,
it was being included by the code using CONFIG_SYS_OMAP24_I2C_SPEED, but it
might not always be the case.

In particular, when introducing I2C SPL support in omap-common's boot-common.c,
the header is missing and including it breaks other devices.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>