powerpc: make 'boot_text_mapped' static

The sparse tool complains as follow:

arch/powerpc/kernel/btext.c:48:5: warning:
 symbol 'boot_text_mapped' was not declared. Should it be static?

This symbol is not used outside of btext.c, so this commit make
it static.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210408011801.557004-3-yukuai3@huawei.com