Fix size address calculation.

Global variables from <asm/sections.h> which are defined
in linker script *.lds files for end addresses has already
a +1 calculation.

For example:
stext = 0x100 with a size about 0x50 will result a etext = 0x150.
In this case a correct size calculation is (etext - stext) = 0x50.

In function 'request_sdram_region' the end address will be
calculated with (start + size - 1) which result a correct
end address of 0x149 in this example.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>