openrisc: Add text patching API support

Add text patching api's to use in subsequent jump_label implementation.  We use
a new fixmap FIX_TEXT_POKE0 entry to temporarily override MMU mappings to allow
read only text pages to be written to.

Previously, __set_fix was marked with __init as it was only used during the
EARLYCON stage. Now that TEXT_POKE mappings require post-init usage
(e.g., FIX_TEXT_POKE0), keeping __init would cause runtime bugs whenset_fixmap
accesses invalid memory. Thus, we remove the __init flag to ensure __set_fix
remains valid beyond initialization.

A new function patch_insn_write is exposed to allow single instruction patching.

Link: https://lore.kernel.org/openrisc/aJIC8o1WmVHol9RY@antec/T/#t



Signed-off-by: chenmiao <chenmiao.ku@gmail.com>
Signed-off-by: Stafford Horne <shorne@gmail.com>