realtek: convert to generic machine initialization

realtek: convert to generic machine initialization

Upstream has implemented MACH_REALTEK_RTL with the MIPS generic
startup framework. Until now OpenWrt disables that with a patch
and instead uses dedicated legacy startup sources.

Convert to upstream startup and take over the missing downstream
bits from the legacy code. Due to the complex nature of this patch
only focus on board-realtek.c for now. Moving other pieces around
(like headers) needs to be done in a separate patch.

Link: https://github.com/openwrt/openwrt/pull/24390
Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>