mx28: Rename regs-power.h to regs-power-mx28.h

The i.MX23 has different register layout and bit placement in the
power supply. Thus, in order to be able to add support for MX23,
rename the MX28's regs-power.h to regs-power-mx28.h . Moreover,
add ifdef around inclusion of regs-*-mx28.h in imx-regs.h so the
MX23 boards will include proper set of registers.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>