omap: serial: fix non-empty uart fifo read abort

OMAP3xxx and OMAP4430 UART IP blocks have a restriction wrt RX FIFO.
Empty RX fifo read causes an abort.

OMAP3xxx:
	UART IP revision >= 0x52 have this issue
	MVR register format is:
	Bits  Field Name  Description  				Type  Reset
	31:8   RESERVED 					RO  	0x0
	7:4    MAJOR  	Major revision number of the module.  	RO  	0x--
	3:0    MINOR  	Minor revision number of the module.  	RO  	0x--

OMAP4xxx:
	All revisions have this issue
	Revision id check is not used as the format of MVR resigster has changed
	For omap4 MVR register reads as: 0x50410602 => Revision id = 0x0602
	Format of MVR register on omap4 is: (Courtesy: Cousson, Benoit)
	Bits  Field Name  Description  				Type  Reset
	31:30 SCHEME  	Scheme revision number of module  	RO  	0x1
	29:28 RESERVED   					RO  	0x1
	27:16 FUNC  	Function revision number of module  	RO  	0x041
	15:11 RTL  		Rtl revision number of module  	RO  	0x00
	10:8  MAJOR 	Major revision number of the module.  	RO  	0x6
	7:6   CUSTOM  	Custom revision number of the module.  	RO  	0x0
	5:0   MINOR  	Minor revision number of the module.  	RO  	0x02

Override the default 8250 read handler: mem_serial_in()
by a custom handler: serial_in_8250()
which makes sure that RX fifo is not read when empty

tested on zoom3(3630) board

Cc: Benoit Cousson <b-cousson@ti.com>
Signed-off-by: Vikram Pandita <vikram.pandita@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>