spi: Move 16MiB partitioning up into spi_chip_read()

We enforced a 16MiB limit in spi_read_chunked() for multi-die flash
chips that can't be fully read at once. The same limit can be useful
for dediprog programmers. So move it into a more generic place.

Change-Id: Iab1fd5b2ea550b4b3ef3e8402e0b6ca218485a51
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/33613
Reviewed-by: Ryan O'Leary
Reviewed-by: ron minnich <rminnich@gmail.com>
Reviewed-by: David Hendricks <david.hendricks@gmail.com>

flash.h

@@ -37,6 +37,12 @@
 #include "libflashrom.h"
 #include "layout.h"
 
+#define KiB (1024)
+#define MiB (1024 * KiB)
+
+/* Assumes `n` and `a` are at most 64-bit wide (to avoid typeof() operator). */
+#define ALIGN_DOWN(n, a) ((n) & ~((uint64_t)(a) - 1))
+
 #define ERROR_PTR ((void*)-1)
 
 /* Error codes */