[new uImage] Cleanup do_botm_linux() boot allocations

This patch moves common pre-boot allocation steps shared between PPC
and M68K to a helper routines:

common:
- get_boot_sp_limit()
- get_boot_cmline()
- get_boot_kbd()

platform:
- set_clocks_in_mhz()

Signed-off-by: Marian Balakowicz <m8@semihalf.com>