Revert "am335x_evm.h: If mmcdev and bootpart switch to mmcdev 1, so should mmcroot."

Upon further inspection and review and chatting with kernel folks, what
happens here is that what mmcblk# a device gets is based on probe order.
So a system with an SD card inserted with place eMMC on mmcblk1, but
without an SD card, it will be on mmcblk0.  So U-boot can only provide a
best guess.  In this case, if no SD card is present, we would want to
pass mmcblk0p2 still.  If an SD card is present, it woudl be able to
provide a uEnv.txt that would be loaded (even if the kernel is NOT
there) which can still update mmcroot variable.

This reverts commit 827512fb.

Cc: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Tom Rini <trini@ti.com>