mtd: st_spi_fsm: Obtain and use EMI clock

ST's Common Clk Framework is now available. This patch ensures the FSM
makes use of it by obtaining and enabling the EMI clock. If system fails
to provide the EMI clock, we bomb out.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>