parisc: Enable the ro_after_init feature

This patch modifies the initial page mapping functions in the following way:

During bootup the init, text and data pages will be mapped RWX and if
supported, with huge pages.

At final stage of the bootup, the kernel calls free_initmem() and then all
pages will be remapped either R-X (for text and read-only data) or RW- (for
data). The __init pages will be dropped.

This reflects the behaviour of the x86 platform.

Signed-off-by: Helge Deller <deller@gmx.de>