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

If, in CONFIG_BOOTCOMMAND, the environment switches both the mmcdev
and bootpart variables to refer to MMC device 1, it would make sense
that the mmcroot env variable should switch to that device as well.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>