fix(bl1): invalidate SP in data cache during secure SMC

Invalidate the SP holding `smc_ctx_t` prior to enabling the data cache
when handling SMCs from the secure world. Enabling the data cache
without doing so results in dirty data either being evicted into main
memory, or being used directly from bl1. This corrupted data causes
system failure as the SMC handler attempts to use it.

Change-Id: I5b7225a6fdd1fcfe34ee054ca46dffea06b84b7d
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>