powerpc/vdso: Mark the vDSO code read-only after init

VDSO text is fixed-up during init so it can't be const,
but it can be read-only after init.

Do the same as x86 in commit 018ef8dc ("x86/vdso: Mark the vDSO
code read-only after init") and arm in commit 11bf9b86 ("ARM/vdso:
Mark the vDSO code read-only after init"), move it into
ro_after_init section.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/e9892d288b646cbdfeef0b2b73edbaf6d3c6cabe.1734174500.git.christophe.leroy@csgroup.eu