drm/panthor: Explicitly include mm.h for the {virt, __phys)_to_pfn() defs

Something on arm[64] must be including <asm/page.h>, but things fail
to compile on sparc64. Make sure this header is included (through
linux/mm.h) so this driver can be compile-tested on all supported
architectures.
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202403031142.Vl4pW7X6-lkp@intel.com/Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240304090812.3941084-3-boris.brezillon@collabora.com

drivers/gpu/drm/panthor/panthor_device.c

@@ -4,6 +4,7 @@
 /* Copyright 2023 Collabora ltd. */
 
 #include <linux/clk.h>
+#include <linux/mm.h>
 #include <linux/platform_device.h>
 #include <linux/pm_domain.h>
 #include <linux/pm_runtime.h>