src: Remove unused 'include <string.h>'

Unused includes found using following commande:
diff <(git grep -l '#include <string.h>' -- src/) <(git grep -l 'memcpy\|memmove\|memset\|memcmp\|memchr\|strdup\|strconcat\|strnlen\|strlen\|strchr\|strncpy\|strcpy\|strcmp\|strncmp\|strspn\|strcspn\|atol\|strrchr\|skip_atoi\|STRINGIFY' -- src/) |grep -v vendorcode |grep '<'

Change-Id: Ibaeec213b6019dfa9c45e3424b38af0e094d0c51
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41242
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Wim Vervoorn <wvervoorn@eltan.com>

src/drivers/spi/atmel.c

@@ -4,7 +4,6 @@
 #include <commonlib/helpers.h>
 #include <spi_flash.h>
 #include <spi-generic.h>
-#include <string.h>
 
 #include "spi_flash_internal.h"