ARM: OMAP2+: Remove unused platform init code and headers

After converting mach-omap2 to boot with device tree only,
we can now remove some more unused platform init code and
headers:

- omap4-keypad related functions are no longer needed with
  with device tree

- board-rx51.h and hsmmc.h are now unused, and board-rx51.c
  has some unneded headers

- serial.h is no longer needed

- i2c_dev_attr is now set in the driver based on the
  compatible flag, and fifo_depth is unused

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