Introduce ARCH_REALTEK Kconfig option also for 32-bit Arm.

Override the text offset to cope with boot ROM occupying first 0xa800
bytes and further reservations up to 0xf4000 (compare Device Tree).

Add a custom machine_desc to enforce memory carveout for I/O registers.
Signed-off-by: NAndreas Färber <afaerber@suse.de>