logging: mipi_syst: Add CONFIG_LOG_MIPI_SYST_DISABLE_TIMESTAMP

This adds user configurable CONFIG_LOG_MIPI_SYST_DISABLE_TIMESTAMP
Kconfig option to use it for macro MIPI_SYST_PCFG_ENABLE_TIMESTAMP.
Moving macro definition of MIPI_SYST_PCFG_ENABLE_TIMESTAMP from
mipi-sys-t library to platform.h which defaulted to undef.
CONFIG_LOG_MIPI_SYST_DISABLE_TIMESTAMP gives flexibility to users
when needn't use mipi-sys-t protocol timestamp.

Signed-off-by: Fang Huang <fang.huang@intel.com>