target-mips: enable access to DSP ASE if implemented

compute_hflags() will reset DSP h-flags, so MX bit should be initially set
for usermode in cpu_state_reset() if DSP ASE is implemented.
This change will bring back user-mode support for DSP ASE, since one of the
recent changes broke it.

Signed-off-by: Petar Jovanovic <petarj@mips.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>