nvme: Add NVM Express driver support

NVM Express (NVMe) is a register level interface that allows host
software to communicate with a non-volatile memory subsystem. This
interface is optimized for enterprise and client solid state drives,
typically attached to the PCI express interface.

This adds a U-Boot driver support of devices that follow the NVMe
standard [1] and supports basic read/write operations.

Tested with a 400GB Intel SSD 750 series NVMe card with controller
id 8086:0953.

[1] http://www.nvmexpress.org/resources/specifications/

Signed-off-by: Zhikang Zhang <zhikang.zhang@nxp.com>
Signed-off-by: Wenbin Song <wenbin.song@nxp.com>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/Kconfig b/drivers/Kconfig
index 2e03133..613e602 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -50,6 +50,8 @@
 
 source "drivers/net/Kconfig"
 
+source "drivers/nvme/Kconfig"
+
 source "drivers/pci/Kconfig"
 
 source "drivers/pcmcia/Kconfig"