m68k: Calculate THREAD_SIZE from THREAD_SIZE_ORDER

Current THREAD_SIZE_ORDER implementation is not generic.

Improve it by:
  - Defining THREAD_SIZE_ORDER based on the specific platform config,
  - Calculating THREAD_SIZE from THREAD_SIZE_ORDER.
Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Dawei Li <dawei.li@shingroup.cn>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20240304085455.125063-1-dawei.li@shingroup.cnSigned-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/include/asm/thread_info.h

@@ -12,14 +12,15 @@
  */
 #if PAGE_SHIFT < 13
 #ifdef CONFIG_4KSTACKS
-#define THREAD_SIZE	4096
+#define THREAD_SIZE_ORDER	0
 #else
-#define THREAD_SIZE	8192
+#define THREAD_SIZE_ORDER	1
 #endif
 #else
-#define THREAD_SIZE	PAGE_SIZE
+#define THREAD_SIZE_ORDER	0
 #endif
-#define THREAD_SIZE_ORDER	((THREAD_SIZE / PAGE_SIZE) - 1)
+
+#define THREAD_SIZE	(PAGE_SIZE << THREAD_SIZE_ORDER)
 
 #ifndef __ASSEMBLY__