powerpc: Fix preserved memory size for int-vectors

The first 32k of memory is reserved for interrupt vectors, however for
powerpc64 this might not be enough. Fix this by reserving the maximum
size between 32k and the real size of interrupt vectors.

Signed-off-by: GUO Zihua <guozihua@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240113080509.1598290-1-guozihua@huawei.com