LoongArch: Fix build errors for tinyconfig

Building loongarch:tinyconfig fails with the following error.

./arch/loongarch/include/asm/page.h: In function 'pfn_valid':
./arch/loongarch/include/asm/page.h:42:32: error: 'PHYS_OFFSET' undeclared

Add the missing include file and fix succeeding vdso errors.

Fixes: 09cfefb7 ("LoongArch: Add memory management")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>