csky: pgtable.h: Coding convention

C-SKY page table attributes only have 'Dirty' and 'Valid' to
emulate 'PRESENT, READ, WRITE, EXEC, DIRTY, ACCESSED'.

This patch cleanup unnecessary definition.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Arnd Bergmann <arnd@arndb.de>