arm64: introduce an order for exceptions

Lets define an order for masking and unmasking exceptions. 'dai' is
memorable and effectively what we have today.

Masking debug exceptions should cause all other exceptions to be masked
too. Masking SError should mask irq, but not debug exception. Masking
irqs has no side effects for other flags. Keeping to this order makes it
easier for entry.S to know which exceptions should be unmasked.

FIQ is never expected, but we mask it when we mask debug exceptions, and
unmask it at all other times.

Given masking debug exceptions masks everything, we don't need macros
to save/restore that bit independently. Remove them and switch the last
caller over to {en,dis}able_daif().

Signed-off-by: James Morse <james.morse@arm.com>