soc: qcom: enable smsm/smp2p modular build

The newly added smp2p and smsm drivers cannot be loadable modules
but depend on smem, which can be, and that causes a link error:

drivers/built-in.o: In function `qcom_smp2p_intr':
:(.text+0xa6e68): undefined reference to `qcom_smem_get'
drivers/built-in.o: In function `qcom_smp2p_probe':
:(.text+0xa7320): undefined reference to `qcom_smem_alloc'
:(.text+0xa736c): undefined reference to `qcom_smem_get'
drivers/built-in.o: In function `qcom_smsm_probe':
:(.text+0xa7b34): undefined reference to `qcom_smem_get'

This marks all the drivers as 'tristate' to make the Kconfig
dependency resolution work properly.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: dbb04bd7122f ("soc: qcom: smp2p: Qualcomm Shared Memory Point to Point")
Fixes: d7387fc6add4 ("soc: qcom: smsm: Add driver for Qualcomm SMSM")
Signed-off-by: Andy Gross <agross@codeaurora.org>