powerpc/32s: Allocate one 256k IBAT instead of two consecutives 128k IBATs

Today we have the following IBATs allocated:

	---[ Instruction Block Address Translation ]---
	0: 0xc0000000-0xc03fffff 0x00000000         4M Kernel   x     m
	1: 0xc0400000-0xc05fffff 0x00400000         2M Kernel   x     m
	2: 0xc0600000-0xc06fffff 0x00600000         1M Kernel   x     m
	3: 0xc0700000-0xc077ffff 0x00700000       512K Kernel   x     m
	4: 0xc0780000-0xc079ffff 0x00780000       128K Kernel   x     m
	5: 0xc07a0000-0xc07bffff 0x007a0000       128K Kernel   x     m
	6:         -
	7:         -

The two 128K should be a single 256K instead.

When _etext is not aligned to 128Kbytes, the system will allocate
all necessary BATs to the lower 128Kbytes boundary, then allocate
an additional 128Kbytes BAT for the remaining block.

Instead, align the top to 128Kbytes so that the function directly
allocates a 256Kbytes last block:

	---[ Instruction Block Address Translation ]---
	0: 0xc0000000-0xc03fffff 0x00000000         4M Kernel   x     m
	1: 0xc0400000-0xc05fffff 0x00400000         2M Kernel   x     m
	2: 0xc0600000-0xc06fffff 0x00600000         1M Kernel   x     m
	3: 0xc0700000-0xc077ffff 0x00700000       512K Kernel   x     m
	4: 0xc0780000-0xc07bffff 0x00780000       256K Kernel   x     m
	5:         -
	6:         -
	7:         -

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ab58b296832b0ec650e2203200e060adbcb2677d.1637930421.git.christophe.leroy@csgroup.eu