linux: disable TI and Whiteheat serial drivers

The firmware for these drivers breaks parallel make install with recent
versions of make:

| make[1]: *** No rule to make target
`/build/v2013.12/build/tmp-angstrom_v2013_12-eglibc/work/olinuxino_a20-angstrom-linux-gnueabi/linux/3.4.79-r0/image/lib/firmware/./',
needed by
`/build/v2013.12/build/tmp-angstrom_v2013_12-eglibc/work/olinuxino_a20-angstrom-linux-gnueabi/linux/3.4.79-r0/image/lib/firmware/ti_3410.fw'.
Stop.

Since both of them are rarely used and virtually never on ARM systems
just disable them.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
linux: enable cpufreq for olinuxino-a13

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
linux: import EHCI defconfig changes from upstream for olinuxino-a20

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>