Barebox' PBL is able to initialize SoC's memory controller,
but it can be used only if PBL runs from ROM or on-chip SRAM.
MIPS architecture standard boot vector is 0xbfc00000
so on most MIPS SoCs all addresses higher than 0xbfc00000
belong to boot ROM or on-chip SRAM. Thus there's a
simple criterion to check if PBL runs from ROM: just
check if current PC is higher than 0xbfc00000.
Some MIPS boards have ROM start address lower than 0xbfc00000
so it's reasonable to make ROM start address checking board-dependant.

The pbl_blt macro checks if current pc is lower than
the first argument (ROM start address). If so then
next instruction executed is defined by the second argument
of the macro.

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Tested-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>