rt2x00: rt2800lib: add support for RT3352 with 20MHz crystal

On Rt3352 the driver needs to know the frequency of an external
crystal which can be either 40 MHz (as on all other WiSoCs until now)
or 20 MHz.
Get the clock attached by ramips WiSoC platform code which probes
SYSC_REG_SYSCFG (added by John Crispin in commit 6ac8579b) and
introduce a new flag clk_is_20mhz in struct hw_mode_spec to make the
driver aware and use either 40 MHz or 20 MHz specific rf_vals on those
WiSoC platforms.
The introduced support for boards with a 20 MHz crystal is also needed
for RT5350.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Mathias Kresin <dev@kresin.me>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>