Currently u-boot stack is defined at the beginning of MSMC RAM.
This is a problem for uart boot mode as ROM downloads directly to
starting of MSMC RAM.
Fixing it by moving stack to the end of u-boot section and shifting
SYS_TEXT_BASE to the start of MSMC RAM.
Updated division of MSMC RAM is shown below:
	-----------------------------------------
	|		|	|		|
	| U-Boot text	|U-Boot	| SPL text	|
	| download	| Stack	| Download +	|
	|		|	| SPL_BSS +	|
	|		|	| SPL_STACK	|
	-----------------------------------------
	[1]		[2]	[3]		[4]

[1] SYS_TEXT_BASE (Start of MSMC RAM)
[2] SPL_TEXT_BASE - GBL_DATA_SIZE
[3] SPL_TEXT_BASE
[4] END of SPL

[1] + [2] is at least 1M on all platforms, so no chance of overlap.

Reviewed-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>