powerpc/ps3: Add PS3 repository write support

Add a new config option CONFIG_PS3_REPOSITORY_WRITE that
conditionally builds in support to create, write and delete
nodes in the PS3 system repository.

This support will allow Linux based bootloaders to manage data
in the system repository for use by later boot stages,

Signed-off-by: Geoff Levand <geoff@infradead.org>