omap2+: Use Kconfig symbol in Makefile instead of obj-y

As noted by Grant Likely <grant.likely@secretlab.ca>, omap2+ Makefile unnecessarily
repeats entries for common device init code instead of using Kconfig symbol.

Remove references to hsmmc.o and board-flash.o. Also omap_phy_internal.o
references can be removed once it has some Kconfig symbol to use.

Signed-off-by: Tony Lindgren <tony@atomide.com>