powerpc/fsl_booke: Enable STRICT_KERNEL_RWX

Enable STRICT_KERNEL_RWX on fsl_booke.

For that, we need additional TLBCAMs dedicated to linear mapping,
based on the alignment of _sinittext.

By default, up to 768 Mbytes of memory are mapped.
It uses 3 TLBCAMs of size 256 Mbytes.

With a data alignment of 16, we need up to 9 TLBCAMs:
  16/16/16/16/64/64/64/256/256

With a data alignment of 4, we need up to 12 TLBCAMs:
  4/4/4/4/16/16/16/64/64/64/256/256

With a data alignment of 1, we need up to 15 TLBCAMs:
  1/1/1/1/4/4/4/16/16/16/64/64/64/256/256

By default, set a 16 Mbytes alignment as a compromise between memory
usage and number of TLBCAMs. This can be adjusted manually when needed.

For the time being, it doens't work when the base is randomised.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/29f9e5d2bbbc83ae9ca879265426a6278bf4d5bb.1634292136.git.christophe.leroy@csgroup.eu