include: sd: revise the minimum alignment of card_buffer to 4-bytes

Since a85ffa8, `struct sd_card.card_buffer` has naturally fallen at an
offset that is unaligned. This meant that on systems without support for
unaligned access (e.g: Cortex-M0), a hard fault would present when
executing code that casts the buffer to `uint32_t` (such as
`sdmmc_spi_read_cxd()`, `card_query_written()`, etc...)

Historically, it appears that the alignment of the `card_buffer` member
was good and operational only by chance.

Altering the default value of `CONFIG_SDHC_BUFFER_ALIGNMENT` was
rejected, as this has wider implications.

Fixes #62619

Signed-off-by: Attie Grande <attie.grande@argentum-systems.co.uk>

include/zephyr/sd/sd.h

@@ -80,8 +80,14 @@ struct sd_card {
 	uint8_t bus_width; /*!< Desired bus width */
 	uint32_t cccr_flags; /*!< SDIO CCCR data */
 	struct sdio_func func0; /*!< Function 0 common card data */
+
+	/* NOTE: The buffer is accessed as a uint32_t* by the SD subsystem, so must be
+	 * aligned to 4 bytes for platforms that don't support unaligned access...
+	 * Systems where the buffer is accessed by DMA may require wider alignment, in
+	 * which case, use CONFIG_SDHC_BUFFER_ALIGNMENT.
+	 */
 	uint8_t card_buffer[CONFIG_SD_BUFFER_SIZE]
-		__aligned(CONFIG_SDHC_BUFFER_ALIGNMENT); /* Card internal buffer */
+		__aligned(MAX(4, CONFIG_SDHC_BUFFER_ALIGNMENT)); /* Card internal buffer */
 };
 
 /**