ath79: read back reset register

ath79: read back reset register

Read back the reset register in order to flush the cache. This fixes
spurious reboot hangs on TP-Link TL-WDR3600 and TL-WDR4300 with Zentel
DRAM chips.

This issue was fixed in the past, but switching to the reset-driver
specific implementation removed the cache barrier which was previously
implicitly added by reading back the register in question.

Link: https://github.com/freifunk-gluon/gluon/issues/2904
Link: https://github.com/openwrt/openwrt/issues/13043
Link: https://dev.archive.openwrt.org/ticket/17839
Link: f8a7bfe1cb2c ("MIPS: ath79: fix system restart")

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 2fe8ecd880396b5ae25fe9583aaa1d71be0b8468)