ARMv7: Alias dmbld() to dmb()

'dmb ld' is not a recognized instruction for ARMv7. Since generic code
may use 'dmb ld', alias it to 'dmb' when building for ARMv7.

Change-Id: I502f360cb6412897ca9580b725d9f79469a7612e
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>