realtek: align kernel config with upstream

realtek: align kernel config with upstream

Since the start of the Realtek target OpenWrt works with RTL83XX as the
target architecture. Upstream is using MACH_REALTEK_RTL instead. To
simplify further development align that.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/16963
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 65964c42f8744ed8ca09448a155b99e3a99be283)
Link: https://github.com/openwrt/openwrt/pull/17097
Signed-off-by: Petr Štetiar <ynezz@true.cz>