lib: rational: copy the rational fraction lib routines from Linux

Copy the best rational approximation calculation routines from Linux.
Typical usecase for these routines is to calculate the M/N divider
values for PLLs to reach a specific clock rate.

This is based on linux kernel commit:
"lib/math/rational.c: fix possible incorrect result from rational
fractions helper"
(sha1: 323dd2c3ed0641f49e89b4e420f9eef5d3d5a881)

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Tero Kristo <kristo@kernel.org>
diff --git a/lib/Kconfig b/lib/Kconfig
index 15019d2..ad0cd52 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -674,6 +674,13 @@
 	  See also SMBIOS_SYSINFO which allows SMBIOS values to be provided in
 	  the devicetree.
 
+config LIB_RATIONAL
+	bool "enable continued fraction calculation routines"
+
+config SPL_LIB_RATIONAL
+	bool "enable continued fraction calculation routines for SPL"
+	depends on SPL
+
 endmenu
 
 config ASN1_COMPILER