arm64: Move PTE_RDONLY bit handling out of set_pte_at()

Currently PTE_RDONLY is treated as a hardware only bit and not handled
by the pte_mkwrite(), pte_wrprotect() or the user PAGE_* definitions.
The set_pte_at() function is responsible for setting this bit based on
the write permission or dirty state. This patch moves the PTE_RDONLY
handling out of set_pte_at into the pte_mkwrite()/pte_wrprotect()
functions. The PAGE_* definitions to need to be updated to explicitly
include PTE_RDONLY when !PTE_WRITE.

The patch also removes the redundant PAGE_COPY(_EXEC) definitions as
they are identical to the corresponding PAGE_READONLY(_EXEC).

Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>