[MTD] [MAPS] Extend plat-ram to support a supplied probe type

This enhances plat-ram to take a map_probes argument in
the platform_data structure which allow plat-ram to support
any direct-mapped device that MTD supports (jedec, cfi, amd ..)

A few items are also fixed:
- Don't panic if probes is 0
- Actually use the partition list that is passed in

Signed-off-by: Florian Fainelli <florian.fainelli@telecomint.eu>
Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>