eal: split atomic operations to architecture specific

eal: split atomic operations to architecture specific

This patch first adds architecture specific directories to eal.
Then split the atomic operations to architecture specific and generic files.
Architecture specific files are put into the corresponding architecture
directory and common header are put into generic directory.

Update documentation generation with new generic/ directory.

Signed-off-by: Chao Zhu <bjzhuc@cn.ibm.com>
Signed-off-by: David Marchand <david.marchand@6wind.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>