sdmmagpie: Add initial config option and documentation

Add option "ARCH_SDMMAGPIE" to enable compilation for sdmmagpie
target. Add compatible string "sdmmagpie" into the documentation.

Change-Id: I6649c946198394122d2a6cc861e2882b1dbbc529
Signed-off-by: Srinivas Ramana <sramana@codeaurora.org>