video: fbdev: offb: Include missing linux/platform_device.h

A lot of drivers were getting platform and of headers
indirectly via headers like asm/pci.h or asm/prom.h

Most of them were fixed during 5.19 cycle but a newissue was
introduced by commit 52b1b46c ("of: Create platform devices
for OF framebuffers")

Include missing platform_device.h to allow cleaning asm/pci.h

Fixes: 52b1b46c

 ("of: Create platform devices for OF framebuffers")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Helge Deller <deller@gmx.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/f75b383673663e27f6b57e50b4abfb9fe3780b00.1657264228.git.christophe.leroy@csgroup.eu