powerpc: pseries/dtl.c should include asm/firmware.h

A randconfig build on powerpc failed with:

dtl.c: In function 'dtl_init':
dtl.c:238: error: implicit declaration of function 'firmware_has_feature'
dtl.c:238: error: 'FW_FEATURE_SPLPAR' undeclared (first use in this function)

- We need firmware.h for these definitions.

Signed-off-by: Sachin Sant <sachinp@in.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>