regcache-rbtree: Fix reg_stride != 1

There are a couple of calculations, which convert between register addresses and
block indices, in regcache_rbtree_sync() and regcache_rbtree_node_alloc() which
assume that reg_stride is 1. This will break the rb cache for configurations
which do not use a reg_stride of 1.

Also rename 'base' in regcache_rbtree_sync() to 'start' to avoid confusion with
'base_reg'.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>