Add buffer_to/from_fifo32(_prefix) helpers

Many peripheral drivers across different SoCs regularly face the same
task of piping a transfer buffer into (or reading it out of) a 32-bit
FIFO register. Sometimes it's just one register, sometimes a whole array
of registers. Sometimes you actually transfer 4 bytes per register
read/write, sometimes only 2 (or even 1). Sometimes writes need to be
prefixed with one or two command bytes which makes the actual payload
buffer "misaligned" in relation to the FIFO and requires a bunch of
tricky bit packing logic to get right. Most of the times transfer
lengths are not guaranteed to be divisible by 4, which also requires a
bunch of logic to treat the potential unaligned end of the transfer
correctly.

We have a dozen different implementations of this same pattern across
coreboot. This patch introduces a new family of helper functions that
aims to solve all these use cases once and for all (*fingers crossed*).

Change-Id: Ia71f66c1cee530afa4c77c46a838b4de646ffcfb
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34850
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

payloads/libpayload/include/libpayload.h

@@ -445,6 +445,25 @@ static inline int __ffs(u32 x) { return log2(x & (u32)(-(s32)x)); }
 /** @} */
 
 
+/**
+ * @defgroup mmio MMIO helper functions
+ * @{
+ */
+#if !CONFIG(LP_ARCH_MIPS)
+void buffer_from_fifo32(void *buffer, size_t size, void *fifo,
+			int fifo_stride, int fifo_width);
+void buffer_to_fifo32_prefix(void *buffer, u32 prefix, int prefsz, size_t size,
+			     void *fifo, int fifo_stride, int fifo_width);
+static inline void buffer_to_fifo32(void *buffer, size_t size, void *fifo,
+				    int fifo_stride, int fifo_width)
+{
+	buffer_to_fifo32_prefix(buffer, size, 0, 0, fifo,
+				fifo_stride, fifo_width);
+}
+#endif
+/** @} */
+
+
 /**
  * @defgroup hash Hashing functions
  * @{