drivers/spi: add drivers for sdcard mounted on the spi bus

Currently supports initialization, read, write, and erase operations.
Tested on HiFive Uneashed

implementation follows SD association's SPI access protocol, found
as doc http://t.cn/AiB8quFZ

Change-Id: I464d2334b8227e448c1c7e324c0455023cffb72a
Signed-off-by: Xiang Wang <merle@hardenedlinux.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35118
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

0 files changed, 0 insertions, 0 deletions