MIPS: ralink: mt7620: Improve clock frequency detection

MIPS: ralink: mt7620: Improve clock frequency detection

The current code assumes that the peripheral clock always
runs at 40MHz which is not true in all configuration. The
peripheral clock can also use the reference clock instead
of the fixed 40MHz rate. If the reference clock runs at a
different rate, various peripheries are behaving incorrectly.

Additionally, the currectly calculated system clock is also
wrong. The actual value what the code computes is the rate
of the DRAM which can be different from the system clock.

Add new helper functions to get the rate of the different
clocks and use the correct values for the registered clock
devices.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5755/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>