Memory: Remove count of cached pages and add InvalidateRegion

In a future commit, the count of cached pages will be reintroduced in
the actual surface cache. Also adds an Invalidate only to the cache
which marks a region as invalid in order to try to avoid a costly flush
from 3ds memory

src/tests/core/arm/arm_test_common.cpp

@@ -20,7 +20,6 @@ TestEnvironment::TestEnvironment(bool mutable_memory_)
 
     page_table->pointers.fill(nullptr);
     page_table->attributes.fill(Memory::PageType::Unmapped);
-    page_table->cached_res_count.fill(0);
 
     Memory::MapIoRegion(*page_table, 0x00000000, 0x80000000, test_memory);
     Memory::MapIoRegion(*page_table, 0x80000000, 0x80000000, test_memory);