test/py: support 'memstart =' in u_boot_utils.find_ram_base()

Some archs like MIPS or PPC have a different 'bdinfo' output
than ARM regarding the memory configuration. Also support
'memstart = 0x*' in u_boot_utils.find_ram_base() to make
all tests requiring the RAM base working on those archs.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Acked-by: Stephen Warren <swarren@nvidia.com>