soc/intel/{baytrail,braswell,broadwell}: 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: Idc1a8a93a779f92079a0fbbcbc63530ffc061112
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50692
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/soc/intel/broadwell/pch/lpc.c

@@ -11,7 +11,6 @@
 #include <arch/ioapic.h>
 #include <acpi/acpi.h>
 #include <cpu/x86/smm.h>
-#include <string.h>
 #include <soc/iobp.h>
 #include <soc/iomap.h>
 #include <soc/lpc.h>