soc/intel/elkhartlake: Remove unused <string.h>

Found using:
diff <(git grep -l '#include <string.h>' -- src/) <(git grep -l 'STRINGIFY\|memcpy\|memmove\|memset\|memcmp\|memchr\|strdup\|strconcat\|strnlen\|strlen\|strchr\|strncpy\|strcpy\|strcmp\|strncmp\|strspn\|strcspn\|strstr\|strtok_r\|strtok\|atol\|strrchr\|skip_atoi\|snprintf' -- src/)|grep '<'

Change-Id: I66a288df5c10c3ee5b9df599edc02aaed9a334e2
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50694
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/soc/intel/elkhartlake/bootblock/report_platform.c

@@ -12,7 +12,6 @@
 #include <soc/bootblock.h>
 #include <soc/pch.h>
 #include <soc/pci_devs.h>
-#include <string.h>
 
 static struct {
 	uint32_t cpuid;

src/soc/intel/elkhartlake/fsp_params.c

@@ -13,7 +13,6 @@
 #include <soc/pci_devs.h>
 #include <soc/ramstage.h>
 #include <soc/soc_chip.h>
-#include <string.h>
 
 /*
  * ME End of Post configuration

src/soc/intel/elkhartlake/romstage/fsp_params.c

@@ -6,7 +6,6 @@
 #include <soc/pci_devs.h>
 #include <soc/romstage.h>
 #include <soc/soc_chip.h>
-#include <string.h>
 
 static void soc_memory_init_params(FSP_M_CONFIG *m_cfg,
 		const struct soc_intel_elkhartlake_config *config)