fix(spmd): fix build error with spmd

fix(spmd): fix build error with spmd

Currently when we build with 'SPD=spmd SPMD_SPM_AT_SEL2=0'
options, this causes a build failure as
'plat_spmd_handle_group0_interrupt' is called irrespective of
'SPMD_SPM_AT_SEL2' usage in 'spmd_group0_interrupt_handler_nwd'

So make 'plat_spmd_handle_group0_interrupt' dummy implementation
available just when spmd is enabled and SPMC_AT_EL3 is disabled.

Change-Id: Iaccd38faab81671c98f9165f318145187dca9bc2
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>