csky: Fixup PTE global for 2.5:1.5 virtual memory

Fixup commit c2d1adfa9a24 "csky: Add memory layout 2.5G(user):1.5G
(kernel)". That patch broke the global bit in PTE.

C-SKY TLB's entry contain two pages:
vpn, vpn + 1 -> ppn0, ppn1

All PPN's attributes contain global bit and final global is PPN0.G
& PPN1.G. So we must keep PPN0.G and PPN1.G same in one TLB's
entry.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>