AVR32: Define PAGE_SHARED

The virtual framebuffer driver needs PAGE_SHARED, which is not defined
on avr32. Define it.
Reported-by: Oliver Zander <ozander@como.com>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>

include/asm-avr32/pgtable.h

@@ -157,6 +157,7 @@ extern struct page *empty_zero_page;
 #define _PAGE_S(x)	_PAGE_NORMAL(x)
 
 #define PAGE_COPY	_PAGE_P(PAGE_WRITE | PAGE_READ)
+#define PAGE_SHARED	_PAGE_S(PAGE_WRITE | PAGE_READ)
 
 #ifndef __ASSEMBLY__
 /*