soc: arm: infineon_xmc: Use cacheable flash address space

The infineon xmc4xxx series has two ways to access flash: one is the
cacheable address space at 0x8000000 which may return pre-fetched/cached
data to reduce flash access latency, the second is non-cached space
at 0xc000000 which is mainly used for write and erase operations.

Currently the LMA is set to the non-cachable address which is not
efficient for executing in place (XIP). Instead use the cacheable
address for the LMA.

Even though the cacheable address is used for the LMA, the J-Link
probe properly figures that it has to use non-cached space for erasing
and writing to flash.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>