Set XTAL frequency to 8 MHz for LM3S6965

This MCU does not support 10 MHz crystal. 8 MHz crystal is used
by the evaluation board emulated by QEMU.
Remove SYSCONRCC_XTAL_10MHZ

Not all MCUs support 10 MHz crystals.
Make it possible to change SSI speed for BSPs

Also explicitly disable SSI before changing its configuration.