arm64: mm: print hexadecimal EC value in mem_abort_decode()

This change prints the hexadecimal EC value in mem_abort_decode(),
which makes it easier to lookup the corresponding EC in
the ARM Architecture Reference Manual.

The commit 1f9b8936 ("arm64: Decode information from ESR upon mem
faults") prints useful information when memory abort occurs. It would
be easier to lookup "0x25" instead of "DABT" in the document. Then we
can check the corresponding ISS.

For example:
Current	info	  	Document
		  	EC	Exception class
"CP15 MCR/MRC"		0x3	"MCR or MRC access to CP15a..."
"ASIMD"			0x7	"Access to SIMD or floating-point..."
"DABT (current EL)" 	0x25	"Data Abort taken without..."
...

Before:
Unable to handle kernel paging request at virtual address 000000000000c000
Mem abort info:
  ESR = 0x96000046
  Exception class = DABT (current EL), IL = 32 bits
  SET = 0, FnV = 0
  EA = 0, S1PTW = 0
Data abort info:
  ISV = 0, ISS = 0x00000046
  CM = 0, WnR = 1

After:
Unable to handle kernel paging request at virtual address 000000000000c000
Mem abort info:
  ESR = 0x96000046
  EC = 0x25: DABT (current EL), IL = 32 bits
  SET = 0, FnV = 0
  EA = 0, S1PTW = 0
Data abort info:
  ISV = 0, ISS = 0x00000046
  CM = 0, WnR = 1

Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: James Morse <james.morse@arm.com>
Acked-by: Mark Rutland <Mark.rutland@arm.com>
Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Signed-off-by: Will Deacon <will@kernel.org>