soc: loongson: add GUTS driver for loongson-2 platforms

The global utilities block controls PCIE device enabling, alternate
function selection for multiplexed signals, consistency of HDA, USB
and PCIE, configuration of memory controller, rtc controller, lio
controller, and clock control.

This patch adds a driver to manage and access global utilities block
for LoongArch architecture Loongson-2 SoCs. Initially only reading SVR
and registering soc device are supported. Other guts accesses, such
as reading firmware configuration by default, should eventually be
added into this driver as well.

Signed-off-by: Yinbo Zhu <zhuyinbo@loongson.cn>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>