remoteproc: stm32: fix building without ARM SMCC

When compile testing this driver without SMCC support enabled,
we get a link error:

drivers/remoteproc/stm32_rproc.o: In function `stm32_rproc_start':
stm32_rproc.c:(.text+0x776): undefined reference to `__arm_smccc_smc'
drivers/remoteproc/stm32_rproc.o: In function `stm32_rproc_stop':
stm32_rproc.c:(.text+0x92c): undefined reference to `__arm_smccc_smc'

Make the actual call to arm_smccc_smc conditional on the Kconfig
symbol controlling its implementation.

Fixes: 13140de0

 ("remoteproc: stm32: add an ST stm32_rproc driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>