am335x: Switch to CONFIG_SKIP_LOWLEVEL_INIT from guarding SPL or NOR_BOOT

am335x: Switch to CONFIG_SKIP_LOWLEVEL_INIT from guarding SPL or NOR_BOOT

In the case of SPL or NOR_BOOT (no SPL involved) we need to include
certain code in the build.  Use !CONFIG_SKIP_LOWLEVEL_INIT rather than
CONFIG_SPL_BUILD || CONFIG_NOR_BOOT to make the code clearer, and to
make supporting XIP QSPI boot clearer in the code.

Signed-off-by: Tom Rini <trini@ti.com>
Reviewed-by: Wolfgang Denk <wd@denx.de>