plat: juno: fix build for !CSS_USE_SCMI_DRIVER

When CSS_USE_SCMI_DRIVER is not defined or set to 0, we get the
following build error.

plat/arm/board/juno/juno_topology.c:16:19: error: ‘CSS_SCMI_PAYLOAD_BASE’ undeclared here (not in a function)
   .scmi_mbx_mem = CSS_SCMI_PAYLOAD_BASE,
                   ^~~~~~~~~~~~~~~~~~~~~
plat/arm/board/juno/juno_topology.c:17:38: error: ‘CSS_SCMI_MHU_DB_REG_OFF’ undeclared here (not in a function)
   .db_reg_addr = PLAT_CSS_MHU_BASE + CSS_SCMI_MHU_DB_REG_OFF,
                                      ^~~~~~~~~~~~~~~~~~~~~~~
                                      CSS_CPU_PWR_STATE_OFF

Fix the error in order to get function legacy SCPI support functional.

Change-Id: I00cb80db9968aa0be546e33a3a682a2db87719be
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>