powerpc: Refactor permission masks used for __P/__S table and kernel memory flags

Prepare a common version of the permission masks that will be based
on _PAGE_NA, _PAGE_RO, _PAGE_ROX, _PAGE_RW, _PAGE_RWX that will be
defined in platform specific headers in later patches.

Put them in a new header pgtable-masks.h which will be included by
platforms.

And prepare a common version of flags used for mapping kernel memory
that will be based on _PAGE_RO, _PAGE_ROX, _PAGE_RW, _PAGE_RWX that
will be defined in platform specific headers.

Put them in unless _PAGE_KERNEL_RO is already defined so that platform
specific definitions can be dismantled one by one.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/d31b59efcdb38e675479563307af891aeadf6ec9.1695659959.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/pgtable-masks.h

+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_POWERPC_PGTABLE_MASKS_H
+#define _ASM_POWERPC_PGTABLE_MASKS_H
+
+#ifndef _PAGE_NA
+#define _PAGE_NA	0
+#define _PAGE_RO	_PAGE_READ
+#define _PAGE_ROX	(_PAGE_READ | _PAGE_EXEC)
+#define _PAGE_RW	(_PAGE_READ | _PAGE_WRITE)
+#define _PAGE_RWX	(_PAGE_READ | _PAGE_WRITE | _PAGE_EXEC)
+#endif
+
+/* Permission flags for kernel mappings */
+#ifndef _PAGE_KERNEL_RO
+#define _PAGE_KERNEL_RO		_PAGE_RO
+#define _PAGE_KERNEL_ROX	_PAGE_ROX
+#define _PAGE_KERNEL_RW		(_PAGE_RW | _PAGE_DIRTY)
+#define _PAGE_KERNEL_RWX	(_PAGE_RWX | _PAGE_DIRTY)
+#endif
+
+/* Permission masks used to generate the __P and __S table */
+#define PAGE_NONE	__pgprot(_PAGE_BASE | _PAGE_NA)
+#define PAGE_SHARED	__pgprot(_PAGE_BASE | _PAGE_RW)
+#define PAGE_SHARED_X	__pgprot(_PAGE_BASE | _PAGE_RWX)
+#define PAGE_COPY	__pgprot(_PAGE_BASE | _PAGE_RO)
+#define PAGE_COPY_X	__pgprot(_PAGE_BASE | _PAGE_ROX)
+#define PAGE_READONLY	__pgprot(_PAGE_BASE | _PAGE_RO)
+#define PAGE_READONLY_X	__pgprot(_PAGE_BASE | _PAGE_ROX)
+
+#endif /* _ASM_POWERPC_PGTABLE_MASKS_H */