brcm2708: disable CONFIG_BCM2835_THERMAL by default

This fixes:
  Thermal sensors on bcm2835 SoC (BCM2835_THERMAL) [N/m/y/?] (NEW) aborted!

This symbol depends on ARCH_BCM2835 so it should be OK to disable it at
brcm2708 target only.

Fixes: 7d32caa61fb0 ("brcm2708: backport upstream sdhost controller driver")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>